abdominal tightening in 3rd trimester?!

Lo
I know is normal to feel tight, i know abt braxton hicks but I think this is different and would like to know if anyone experienced it...what do you thnk about it?!
I get mt stomach hard and it stays like that for hours!! Until I get to lie down or smthg which is only in the evening as I am working
I am 31 weeks and still working full time ( office job) but my days are 12 hrs long!! and noticed whem I am tired mt stomach is just hard and uncomfortable. Been to hospital check because I couldn't feel baby move- they said is because I am tired and my stomach gets hard.
Does anyone else have this?