Severly constipated during 2nd trimester help :(!?

I haven't had a bowel movement in 3 days and now I'm stuck. I haven't been this way for awhile now. I think it might be my prenatals! What can I do to stop this? I've tried sitting in the toilet for an hour and nothing will come out. Is it bad to push? I'm scared my baby will fall out. Lol I know this sounds ridiculous but I'm a first time mom and I've already had a miscarriage so everything freaks me out. If it helps I'm 17 weeks and my baby is healthy according to my midwife and ultrasound tech.