Another possibility, might be that underwriting guidelines for that lender changed. I read an article recently that for some lenders the guidelines changed between when you were pre-approved locally and the paperwork reached the underwriter for final approval. Of course, your loan officer may not have been aware of the changes until after completing your paperwork. I suggest getting this clarified and see if you qualify at all under their new guidelines. (do not pay any additional money)
When you trust the pre-approval you have received from a lender and now you run the risk of losing your earnest money, that's not only a horrible feeling, it also makes you not want to trust anyone else.
Please make sure you get a second opinion with a mortgage provider who is extremely qualified and recommended to you by people you trust. Make certain they explore every financing option available to you. The loan officer who gave you the false pre-approval may not have checked out all your options.
Ask some people you trust whom they would recommend for you to talk with.
Keller Williams Realty~Downtown
I advise you to check with a reputable lender to find out if you are approvable or not. Your realtor should be able to refer you to a good one, or if you like I'd be glad to take a look at your situation.