I remember when I was carrying my first baby, I also had tummy-ache after carrying a heavy box of books (as my office had to move to a new building). I was worried afterwards and went to see the doctor that afternoon. Luckily, the baby was normal. She also mentioned that I might sometimes feel tummy-ache as "支撐子宮既韌帶受到牽扯所以痛", so don't worry!