Lower abdomen hardens

Hi mummies. I am 13.3 wks tdy. Ytd i woke up and felt that my tummy was tighten and when i touched it, there was a bump on the lower abdomen and its hard. after i stood up and lay down again, its not there. today i felt the same thing aft a nap. May i know is that normal and what is it ? is it the uterus ? #1stimemom #pleasehelp