1) It's possible that you are not interested in sex because your IBS makes you feel bad/sick all the time. Most people don't want sex when they feel bad.
2) What's more likely, however, is that your drop in sex drive is related to your pregnancy. It is VERY common for women to be less interested in sex after childbirth. Some of the reasons why:
a) They are the primary caregiver to the baby and are physically exhausted a lot.
b) Vaginal birth and/or an episiotomy may have changed the shape of the vaginia, making sex unstimulating, if not actually painful.
c) Some women (and men) suffer from a kind of post-traumatic stress syndrome from the birth, in which after having had a baby (or seen one born), the affected person can no longer see that place as sexual. If you feel like sex is dirtying up the womb you carried your child in, or if you find yourself thinking about the birth of your child when you try to have sex, then that may be an indicator that you are having a problem reconnecting your reproductive system with your sexual system.
d) A woman's hormones do not resume their normal levels post-baby. When a woman has too little testosterone, she has too little sex drive. Carrying a baby typically supresses a lot of testosterone, and sometimes it fails to recover.
So, analyze yourself. Do you feel tired and/or sick a lot? Then that's probably why you don't want to have sex. If sex is painful or uncomfortable, see your gynocologist. I read an article that in France they actually have vaginal toning exercises that they get women to do in a doctor's office (I think it also included some mild electrical stimulation as well) to recondition the vagina after birth. There may be some exercises that your doctor can recommend to help reshape your vaginia. She can also check to make sure that if someone stitched you back up, they didn't stitch up too far (heard about that happening). A minor surgical procedure may be needed to correct that problem or some other that occurred because of birth.
If you think that you might be having a problem with returning your reproductive track to sexual work, then a therapist can help you. It's not unheard of, so it shouldn't be hard to find someone to help you through your feelings and get things back on track for you.
And, finally, if none of those things really sounds like you, then it's probably hormones (it usually is). See your gyno and ask to have your hormone levels checked. You may need a hormone supplement, or just a different birth control option that helps boost your missing hormones.