There is no reason why your symptoms should get worse.
Most people have only one incident and then get better.
Only a small percentage develop chronic spondy. Please do not worry, it takes a while for the drug to work - likely a month to two months. This is completely normal and to be expected. My advice, give it some time, if in two months you are not much better, go back to the doctor and ask for something else. There are a lot of treatments available.