When I was pregnant with my twins I was helping my husband do some yard work and thought I pulled a muscle. I was in serious pain! Went in to see the Dr and everything looked fine. He said it was was ligaments stretching to carry the weight. I had to be on bed rest for the rest of the week. My advice is to slow down and take it easy, if not bed rest at least relax a few hours a day so it's not so straining for baby.
take easy and hope everything comes out good for you and baby
since your in high risk pregnancy you shouldnt be moving boxes you should call your doctor just to be on the safe side i am 32 weeks and i was moving boxes like 3 days ago since were moving too and and i got the same things cramps i couldnt even move doctor call me in i was staring to get contractions so he gave me the shot to stop them good luck to you and i hope everything comes out fine