What Causes Heavy Bleeding While Lifting Heavy Objects?

I am 5'4" and weigh 187 and am 27 years old. I gave birth 9 weeks ago. I have been bleeding off and on since. At 6 week checkup had an IUD put in and for about 4 to 5 weeks now when I lift anything over 10 pounds I start bleeding again and kinda heavy. Is this normal? This was my second delivery and both vaginal. They are checking to see if I have PCOS because I have several signs of it. Also when I was younger I had heavy bleeding and counldn't figure out why.