Hi - I Have Just Been Switched To Clindamycin Vs

Hi - I have just been switched to Clindamycin vs Keflex for an infected cyst on my neck. How soon should I see improvement, as I was on Keflex for 3 days with no change. It is the size of a walnut and pus is not coming out of it. The doctor would not lance it. What else should I do? I have never had this before and a 45 year old female. Thanks.