Don't sweat it, try another prof. I had one prof make me feel like crap because I got what he called a "shakey" A in his class. I got a BA in another class and that prof was very enthusiastic about writing me a letter. Try to ask professors that you know have dealt with pre-med LORs before. They understand the process and are usually willing to do everything they can. Otherwise you need to convince the prof to write you a letter, which can take some tact and may be impossible in some cases. They prof who called my A shakey came around somewhat after I discussed my extracurriculars and MCAT scores with him. I've got other letters that are strong so I'm not too worried about his letter being a "glower".
If you are still having trouble getting LORs, that is not a good sign. Even at a big state school (like I go to), you should be able to find some people. Moreover, you should have some letters available from extracurriculars (research PIs are great). Even if the schools you are applying to don't ask for such non-school letters, send them anyway. These sorts of people tend to know your personality better and med schools will most likely look at them. And if the PI liked you, they will most likely be your best letters! (As compared to a prof of a class, even with 20 people; unless you are really lucky and hung out with your prof outside of class and/or screwed around with her/him)
