Confused

Me and my husband have been together for 5 years and i am 31 weeks pregnant with our 1st child together. I don't know what to do because we hardly ever have sex now we used to have sex 3 or 4 times a week but now it's sometimes once a week. I don't know if he just isn't attracted to my pregnant body or if he is just to tired from work or if i should be worried that he is cheating. When i asked him about it he said hes just worried about hurting the baby but I don't believe him because he has 2 children from a previous relationship so im pretty sure hes used to it and when we do have sex he doesn't seem worried or extra careful. And now when we do have sex it only lasts for 5 minutes. Am I just over thinking things i dont know what to do