After tuning my brakes a lot (I was learning how) I noticed suddenly my back brake got really stiff. Does't really matter if the cable is loose or not, the lever itself feels really hard to pull when comparing to the front brake. What could cause this? The cable (since I don't mind cycling when it's raining, maybe it's rusty? but I got the bike for like 3 months) or the lever itself?
First off, carefully check the rear brake and compare it to the front. In particular, the "noodle" of V-brakes is quite fragile and can get bent and crimped. Or it can simply get bumped and moved out of place.
Squeeze the brake lever (on the handlebar) and then let go -- does the cable (as it exits the lever) go "limp"? If so, the brake lever itself is probably OK. If not, the lever may be jammed somehow.
Inspect the cable route and look for any crimp in the cable housing.
It's not unusual for a cable housing to rust up and jam the cable, but it shouldn't happen in 3 months, even if the bike is ridden in the rain. More like the sort of thing that happens after 5-10 years.
If the bike is 3-months new and purchased at a bike shop you should take it back for a complementary "tune-up" anyway.