Sex. I think I'm broken.

An
So I am asking this here, because you mamas and mamas-to-be are around the same stage of pregnancy, so will have the most current relevant responses. Thanks in advance!
I am 22+4 now - with our very first -  and I think my husband and I have only had sex two, maybe three times since we conceived. I don't know what is wrong with me, but I am just never in the mood. Previous to getting pregnant, I didn't want it a whole lot, but it would still be at least every few weeks or so. We are both kinda weird like that, neither having too high of a sex drive to begin with.
I feel bad about it sometimes, because obviously my dear husband does still want it here and there, but I just cannot seem to find the motivation. 
Don't get me wrong, I want to get in the mood; sex with my husband is absolutely incredible and always a wonderful bonding experience for us. It just never feels like a good time to do it though. I don't know if I am even explaining this well enough that anyone can understand just what I mean.
My mom told me "when I was about 4 months along, I couldn't get enough!" 
THAT is how I want to feel! Why can't I find the motivation to even want to get in the mood though? 
I have had a completely smooth pregnancy so far though, no nausea or anything, just still tired and sleep a lot during the day and can't sleep at night. Pretty much nocturnal.
Another concern I have is that I plan to have a completely unmedicated, natural birth. If I'm already not having sex and not getting any stretch to my vulva, do you think that I am definitely going to tear during birth?