Im 26+3weeks and hve the same thing , I carried both my other babies to term , it sounds like Braxton Hicks wich is perfectly fine , tightning without pain , u should be ok:-)
Sounds like it may be from the baby moving around. (I'm also on number 5 btw) The way my current is laying, there are times it feels like a contraction for a minute. But it's the way baby is stretching. That may be what you are feeling.