doc's cnt tell me what's wrong with my sister's tonsil?

my 17 year old sister has had VERY swollen tonsils for a few days now. we went to doctors care..and they tested for strep and mono and came up negative. said her white blood cells looked viral..but it could be an abscess and to go to ER. went to ER..dam near EVERY doctor on the floor took a look..said it looked like mono or strep...tested her again..still negative. said it was in the wrong place for an abscess. they gave her some antibiotics and the white spots on the tonsil went a way..but still swollen and a problem. im upset that no one can tell us what's wrong..i dont understand. all the doctors seemed baffled and told us if it doesnt go away by the end of the antibiotics to come back. 10 days. i dont have money to be throwin at these ppl to tell me NOTHING at all. please help...any ideas?