Hi Lotus - welcome!
So you had a recent tick bite with an EM rash. Can you get more doxy - from another Dr.?
It needs to be treated until sign/symptom free PLUS another 4 weeks with a minimum of 6 weeks treatment.
You should be taking 200 mg twice daily of doxy.
If you don't treat it right away - then you're going to be looking at a chronic lyme infection which will take a lot longer to treat.
There are some LLMD's in England - shoot me an email if you'd like the names.
So you're taking 300 mg daily of doxy....go to another Dr. and get more if you can.
You also need to be taking probiotics - everyday - to replenish the gut bacteria that the doxy is killing.
Since you are new here, it would be helpful for you to read through the thread: "New to Lyme?..Start Here!" It's packed full of useful information.
Take a look at the ILADS Lyme Treatment Guidelines - page 18 and 19 for length of treatment and antibiotic choices.
www.lymenet.org/BurrGuide200810.pdf