Painful sex?

This may be TMI but I'm genuinely curious, me and my fiancé never used lube really (didn't need to honestly) before I got pregnant so I know that wouldn't contribute to this, but ever since I got pregnant sex is extremely painful...like someone is shoving a knife in there instead of a penis lol is the only way I can describe it! How would getting pregnant have anything to do with sex all of a sudden hurting like this?