he got another girl pregnant- UPDATE

Found out that the girl he had been cheating on me with before I got pregnant ( didn't find out until I was about 6 weeks ) he got pregnant also. He took her to prom tonight. He had plenty of chances to leave and I even told him to bug off and he still called and called and then told me he did want us and loved us, lol! It was all a big ass lie. For no reason! He cheated on me for over two years and I even ended up in the hospital because of the stress of it all! But man, finding out this? I'm trying to keep myself together but it's real hard. 30 weeks exactly today. Why.. just why? 
Update: she got pregnant right after me. His whole family knows. He never inteneded on telling his family about me or the baby. My heart just aches so much. I haven't been able to eat more than once since I found out. Been throwing up any liquids, doctor gave me some medicine to try and if it doesn't work, she wants me in the hospital. I want to just sleep until he is here and forget who his dad even is. Somehow I'm the one who he can't stand leaving him, but I'm also the one that he wants to die at the thought of his family knowing about me or anyone for that matter. I told his step mom and she told his dad, so they know about me now. finding out the truth this weekend killed me. I had a doctors appointment today and she had me go get a sonogram because she was worried the baby wasn't gaining weight. I was convinced that he was just so small and that's why I don't feel him move as strong anymore. Turns out, baby boy just doesn't have any room to move. He's 2 pounds 11 ounces, and I'm not sure how long. His head is at the bottom of my belly, and his legs are all smooshed into my right rib cage. It hurts pretty bad when he moves and my tummy is always sore, but I had no clue it was because he was a big baby. I'm not really showing, I just look a little chubby. My heart just aches but we'll get through this. Every time I think of me being pregnant, i remember she's going through almost the same thing and I throw up.