Lots of pressure? Terrified.

Hey ladies. I’m currently 15+1- third pregnancy, first to make it past 5 weeks. Today, I had a single episode of diarrhea (which makes me nervous, even though I’ve got IBS and alternated between diarrhea and constipation pre-pregnancy). Then, I started to notice that (especially when I’m laying on my back) I’m feeling a lot of lower abdominal fullness/pressure. I’m not sure what’s normal and what’s not. I don’t want to bother the on-call OB right now (of course I will if I begin cramping or having actual pain) since it’s Sunday night, but every little thing just terrifies me because of my history. Any experience or advice?