I am so sorry that you had the reason to find this site, but so happy that you DID find us for support. I would have to say that your instincts are right on. He's betrayed you with OW (other woman) physically and emotionally. You caught them. He flipped out because his happy fantasy of having you as "the backup plan" and his tootsie for rodgering has just exploded. He's lying to save his ass and re-writing history quickly so that he can justify his immoral behavior.
Deep breaths. Please take a look in the upper left corner, in the yellow box, and click on The Healing Library. Read thru it there is a lot of good stuff in there for you. Read any posting on this forum that has a bulls-eye on it also good information. Take care of yourself eat, stay hydrated, try to rest. You can't take care of your wonderful children unless you take care of yourself first.
You really do need to see a lawyer to find out what your rights are, since you are carrying his child. You need to get paternity established and get everything in place so that you can receive child support ASAP. You may also be entitled to other support since you are pregnant. But only a lawyer can answer those questions for you.
And as compassionately as I can put it, you simply must call today and get a STD/HIV test scheduled tomorrow if possible. You cannot trust that he hasn't been swiving only one person, you have no idea of who the OW has been with as well, and you sure can't trust a lier's word that he actually used protection. Some of these diseases can cross placental barriers, so to protect your unborn child, you need to be seen immediately. I'm so very sorry.
This is a hell of a lot to dump on you. Don't feel badly if you feel overwhelmed, need to read several times, and cry hysterically. We've been there too. Please keep coming back for support we are all of us, here for you. (((hugs)))