TMI diarrhea 33 weeks

So last night around 8pm (almost 2am now) I had to practically run to the toilet and I'm not kidding when I say it was explosive 🙈. I have had it happen again about an hour later, and woke up not long ago with the same problem. Up until this point I have been quite badly constipated so this has come as a big shock.

I have not changed anything in my diet and otherwise feel okay aside from a little stomach ache when it happens..

What can cause this very abrupt change in my bowel movements? Should I be worried? Or just put it down to yet another pregnancy symptom? I am 33 weeks.