I don't think it is normal. Big Moe, the largest of the 4 female betta's that I bought has a HUGE belly. It was like that when I bought her, but it seems to have gotten bigger now.
Here are some pics....you can see another girl in the background of some and all of the other 3 don't have this "belly":

http://img.photobucket.com/albums/v150/grain190/DSC03839.jpg
http://img.photobucket.com/albums/v150/grain190/DSC03838.jpg
http://img.photobucket.com/albums/v150/grain190/DSC03837.jpg
http://img.photobucket.com/albums/v150/grain190/DSC03836.jpg
http://img.photobucket.com/albums/v150/grain190/DSC03833.jpg
http://img.photobucket.com/albums/v150/grain190/DSC03832.jpg
http://img.photobucket.com/albums/v150/grain190/DSC03828.jpg
http://img.photobucket.com/albums/v150/grain190/DSC03826.jpg

sorry, they are blurry.....but that's the best i can do

Any chance of a photo being taken from above as the swelling looks very similer to the type of tumor my male betta had whom eventually I had to put to sleep. Is the bulge more on one side than the other? If it is a tumor unfortunately all you can do is keep her comfortable and let her go when time is right.

She may also be bloated due to constipation is she pooping ?

are they still eating well, how are they swimming? is it possible that they are pregnant?

i will try to get pics from above soon as i can...it is perfectly even on both sides, no asymmetry or anything like that. She is the only one that I have seen eat so far, as the others tend to just stay toward the bottom, and she likes to hang out around the top. I have not noticed poop yet. I will feed them some peas later today just to be sure.

could be constipation then especially if she is getting all the food. If her poop is color of food an long an trailing then its constipation.

actually she is just eggy . Female bettas bellies look like that when they are full of eggs and ready to breed. the girls will get eggy when you feed them really well in fact that is one of things you do to get them eggy to breed.All you need to do is fast her a couple days and she should reabsorb them.

OK, cool. I was hoping it was eggs and not something horrible!!

-do you have any tips on getting food to the others who don't come up top to eat? I have the betta pellets, but they won't come up and get those.
i also have beta flakes that i bought the other day, but i see them just sitting on the bottom of the tank, doesn't look like they're getting eaten.

All my bettas love freeze dried bloodworms and brine shrimp. I accidentally fed too many worms to one of my males and he got a bloated belly and looked miserable for a few days but they passed and he's fine now. I think the shrimp are easier for them to digest. I won't feed live worms or shrimp because of the possibility of parasites.

as mentioned they do love frozen brine shrimp and frozen blood worms. I would only feed the blood worms about three times a week. they are more of a treat. Some bettas will not eat flakes. they should go for the pellets though. Try feeding pellets only for a week or so and they should start eating them. Also try putting the pellets in and if they do not eat in about 10 minutes remove any uneaten pellets and discard them and do not feed them till the next feeding.Don't give in. Keep this up for a week or so ( may only take a couple days) and they should come around and eagerly start eating . While you are trying to get them to accept pellets do not feed them anything else just the pellets. After a couple weeks of pellets you should be able to return to a varied diet of pellets, shrimp, and blood worms. I had a male betta that would not eat when I first got him I tried this routine ( actually excellent advice from MR.Bumblebee) and it only took my betta a couple days to start eating pellets

ok cool. thanks I currently have them eating the flakes, and now they all seem to like them alot. The smallest one doesn't come to the top, but she grabs the ones that are starting to sink. i am going to try the pellets though, they are probably better for them, and i want them to be able to have a good variety. after that, i'll move on to the frozen's!