I have a Specialized Hardrock Sport, around a 2000, 2001 year. It doesn't have many miles on it, but recently the crank started coming loose and clicking when I pedaled. I took it to a bike shop and they tightened it (I'm not very mechanically inclined and just moved into an apartment and don't have many tools with me yet.) It was fine for about 20 minutes, and then it came loose again and started clicking. I took it back, they tightened it a second time. Rode for about 20 more minutes and it's loose yet again. Now, I'm not doing hard riding by any means. I'm just riding around my college campus; I haven't take it on any trails since I've moved. I'm not jumping or bunny hopping or doing anything else harsh either. What are some reasons why the crank would keep coming loose after so little riding? A bent crank? Bad bearings? Thanks.