Why does hurt?

I've been wondering to myslef if I should ask this or not but i've been going through this for awhile I have a 2 1/2 yr old and a 6 month old. Well ever since i've been about 3 months pregnant with my first son it's hurt. My husband was my first and only. It didn't hurt at first and now it's extremely painful for me the word anymore just turns me off. im only 23 and my husband is 26 I've asked the doctor first she said it was because of cysts. Well I don't have them.. Then she told me to use alot of lubricant we've done it used diffrent kinds and it still hurts. I try to get excited before we do it still doesn't work. I love my husband and I am very attracted to him so that's not it. And he's not huge he's not small but he's decent for me... It doesn't hurt when he first goes in just after a certain point. It's to the point where I get in tears and he stops and I feel so bad. Anyone else have this problem or had this problem and knows what might be wrong? Please help!!!!!