Credit score for student loan question.?

Ok.. i have always got my own loans from SallieMae because i've never had a co-signer and my credit is good. Well i applied for my last loan for my last semester of school and i was denied unless i had a co signer. I have no clue why so i called and they said to check my credit report. My report was all positive except 4 medical bills that have been there the whole time. If i were to pay these bills of do you think it was help my score so i can get my last loan? I'm just worried i won't be able to graduate. The last loan i got was in Jan so maybe time has something to do with it and i should wait to apply again. How long will it take for the medical bills to be removed if i pay them? I'm really worried.