Hi Deeply, you should check this post: http://www.howto3000.com/the-answer-of-is-cellulitis-contagious-and-other-aspects-about-cellulitis-infection.html
OTC Domboro or bluboro compress from a pharmacy may also help to alleviate inflammation and swelling of cellulitis. Consult with Dr.
Not a Doc of course, but From what I know, cellulitis is an objective symptom of inflammation caused from some type of infection. Depends on whether the strain of Bacteria causing the infection is contagious. Normally treated with course of antibiotics, but certain bacteria initially resistant to treatment may require alternative or extended treatment. A culture, swab test by a dermatologist or family Doc can determine what type of bacteria is present and best treatment methods. If it is just a localized bacterial infection, like staph for example I wouldnt deem it sexually transmitted. MRSA bacteria is highly contagious but can still be treated. Best to treat early, before symptoms worsen. doesn't usually resolve on its own.