Hi A.,
Congrats on your new LO! While you always should get a real diagnosis from your caregiver, I can tell you that our situations are very similar: I just had my second child in January while also caring for an active 16 month old son. My daughter wasn't as big as yours, but I did have some tearing that required stitches. My bleeding had subsided and then began again at around my 6-week checkup. My doc said that I had opened my stitches and that it was also likely that I was overexerting myself, running around and lifting my son too much (he weighed 30 lbs then) right after delivery. I also have exclusively breastfed, so this is why he didn't think it was my period. Do you have any cramping or back pain? I did, which meant that, although I basically felt fine, I was probably still not completely healed internally. It wasn't until my daughter was about 3 months that the bleeding and brown spots totally went away. I have been on a low-hormone oral birth control since my 6-week checkup and haven't had my period yet.
So, try taking a look at how much you're doing and try to find ways to minimize exertion: make the bag boy carry your groceries to the car and then have your SO or a neighbor carry them to the house, don't try to carry both of your children at once (like I did, LOL), encourage your older child to walk more and help them walk up and down stairs instead of carrying them, drag the laundry down the stairs or better yet, have someone else do it!
Hope this helps!!!