Sometimes that short PP you can have some random bleeding....sometimes just because you overdid it or whatever. It is VERY unlikely for her to be pregnant. Most women who are BF at least every 3 hours during the day and every 5 at night will not be fertile and most don't even get their period back for several months (I BF exclusively around the clock and didn't get anything back till around a year). It is only when the feedings start spacing out and you start giving supplemental food that really have to be concerned. http://www.kellymom.com/bf/normal/fertility.html
I would probably just go to the Dr to have things checked out. It is possible, but I think unlikely that it could be her cycle coming back. Cycles can be REALLY weird when you are first getting them back and you are BF. I think sometimes mine would start and then stop because the hormone the BF creates "turns it off".
And, FWIW, really should not be giving juice this young. There is no real reason to do this and it is backfiring as it unseals the digestive tract and some of the good the BF is doing as far as protecting from disease, esp in the gut. http://www.kellymom.com/nutrition/solids/delay-solids.html
Are you worried that he is not having many BM? That is completely normal in a BF baby because the breast milk is digested so fully there is little waste.
Or is the ped telling you to give iron supplements which may be causing constipation? Those are totally unnecessary. Unfortunately, many peds are not up to date with that information http://www.kellymom.com/nutrition/vitamins/iron.html