Worried. 21 weeks.

This is my 3rd baby (7th pregnancy) and I have a retroverted uturus wich makes my cervix super super low. Like right there when I put my finger inside. (Sorry for tmi) but I had the flu vaccine last Monday and have had really bad flu symptoms since. But what I'm worried about is when I cough I feel a pressure down there. And I'm worried because I herd once you have more than one baby your cervix stays open a cm or two. What if I go in to preterm labour? Also I find it super hard to poop (sorry for tmi again) I mean it's there bit it's like it's stuck.... any ladies who can help. I don't see my midwife for another 4 weeks xx thanks