Swollen right foot. The cause for this.

Dear Sir/ Mam My right foot has been swollen since past 3 days. It started off as the second big toe becoming swollen and then it spread to the big toe and then to the entire feet. I have shown it to two separate doctors, one a general physician and another an orthopaedic. Both of them have me muscle relaxant tablets but it only helps for a temporary period and then the swelling starts becoming painful again. The backside of the second toe has turned slightly black as well. Additionally, I got my blood tested for uric acid and sugar but apart from slightly high cholesterol all other parameters are completely within the normal limits. I don't remember being pricked in that area anytime in the near past or any injury either. It has become painful now and I'm unable to walk properly. Please suggest what could be the cause for this.