He is suffering from leg pain. We do?

My father has high blood pressure & he takes Metacerton 50 & Urimax 0.4 mg regularly. On this 4th January he got high fever & he was given medicine. Different blood tests were done but all were normal except his platelets count. Then after 7 days again test was done & the count of platelets were increased. He was under medication but suddenly he had allergy problem for which doctor told to stop taking all the medicines. Now he is not getting fever  but for the last two days he is getting severe leg pain (from thigh to foot) & he can't move. During this period he is feeling very weak also. We opted hot water bag & some ointment for pain & gave  Calpol 500 for fever & pain.Whatthe reason of this sudden pain? What should we do? He had no injury during this period or before.