Firm uterus???/

So when I found out I was pregnant last july, my uterus was super firm...thats one of the reasons I knew I was pregnant.  I had my baby march 31st...stopeed nursing about 6 weeks later and got my AF on schedule at the end of may - very heavy for about 2 days then pretty light.  Now my uterus feels so hard again and I have gained 7lbs in about 3 weeks....anyone have this happen?  I though it could possiby be scar tissue build up from my 2 sections and panni removal but when I bend over i "feel" soreness in my uterus.....thanks!!