First of all no, the hymen does not grow. The hymen is a thin membrane that partially covers the vaginal opening. It doesn't contain any blood vessels, so tearing it wouldn't cause the blood in your underwear. Since you're not having sex regularly the muscles in your vagina are still very constrict and aren't used to a penis or anything foreign in there. When you fingered yourself you most likely spread the muscle in your vagina, causing it to tear slightly and bleed a bit, if this is the case, then it's normal. Another thing is, depending on how roughly you fingered and/or the length of your nails, you could have scratched the wall of your vagina. However, if the bleeding stopped then you should be fine. If bleeding persists, then seek medical help immediately.
The hymen doesn't grow, but it doesn't completely go away after sex either. When you have sex the hymen actually just stretches and/or tears and hymen remnants are left. So when you finger yourself it is possible to catch part of the hymen and tear it more causing a little bit of bleeding.