pregnant or sick?

I have 2 little female platties in my 6 gallon tank. I bought them beginning of January. One was always chunkier than the other, but now she is so big, it looks uncomfortable, like she may explode any second. Her stomach never gets smaller. She acts totally normal, good appetite, normal poops, active etc... I thought she was pregnant but wouldn't she have given birth by now? I don't see an illness with just a big stomach. I am very puzzled.

Here she is:

How long are they usually pregnant for??

wow! Thanks everyone! I got 2 females cause I didn't want more babies!!! I guess she was already pregnant when I got her... I will put my filter intake cover to good use and put more plants. When I do see them, I will try to take them out and put them with my daughter's baby molly fry in her aquarium. I put a square net in there for them.