I was treated for tonsillitis for a few days (told it should start improving within 3 days), saw a different doctor who then gave me steroids too, which made me feel better for the days I was on them, once finished back to crap again. Different doctor again (our practice has like 10 doctors), who upped the dosage of antibiotics but also ordered blood tests. A couple days later, get a call to say stop taking the antibiotics, I had glandular fever! Didn't initially suspect it because I wasn't tired, just had a sore throat, gross tonsils and swollen glands around my head/throat. So probably took a couple weeks all up for them to work it out.
I'd say it would be worth asking for some bloods to get done to make sure it's not something else.