Recurrent back pain

Hi, I'm 18 and I have a reccurrent pain in my lower left back.I am pretty active but havent injured myself in any visible way.I've been having this back pain that comes and goes, it is in my left hip/lower left back and often goes down my leg sometimes to my foot if its severe.
It seems to come randomly but more often when I've done some leg stretching or sit ups.I can get rid of the pain sometimes by lying on my stomach and raising my shoulders without using my arms which works the lower back muscles, but that is never permanent, at the most the effects of this excercise will last a few hours.

Can anyone give me tips to reduce the pain other then painkillers?Or what I should do/not do to improve this condition?

PS: I will consult a doctor when I go to germany but that is in a few months, here I have to pay 50€ per visit which I simply dont have.So any tips would be appreciated.