Possibly Pregnant, Sort of Lactating, On the Shot

First off shes 16 and so am i... We live together and are on our own and thinking about getting married.

Well my girlfriend started the shot(depo-provera) 2 months ago (august 5). We waited much longer than the doctor said we had to to have unprotected sex. Since we have been having unprotected sex. She spotted really lightly during her normal periods both months... I got concerned the first month so I did some investigating then and it turned up that this is normal is the shot stops ovulation so a period may be really light or stop all together.
Now about 30min ago we were fooling around and we noticed that she had lactated... She told me to taste it and it did not taste like regular breast milk(dont ask how I know)... Is was sour and tasted like regular cow milk... There was very little of it, but she could squeez it out, but no more than a drop. Now here is my question, when would a woman start lactating during a normal pregnancy? Also she has shown no signs of pregnancy. She has shown symptoms from the shot( small weight gain around 2lbs, brown spotting instead of period).
Now, we do know for sure that if a baby was concieved it was right around 2 months ago because we waited 4 days(until she got off her period) we fallowed the directions exactly for the shot...

Oh and one more thing.... She lactated once before but only once and only in one breast and it was very little, and I believe she was on the pill at the time.

Also we had just gotten done working out (very stressful cardio and lots of sweating) so maybe this is a build up of something...?

So is the lactating a sign of pregnancy at 2 months? Would the hormones in the shot make her lactate? Or what would cause this?

Also a general question when do women start lactating?

If you have any questions post and I will get back asap, she has gone to work and I have nothing to do but sit here and worry...