Severe sore throat pain,sore tongue. tiny red spots throughout the mouth. mainly inside on cheeks. dr said bad sinus?could be the methotrexate i take?

Mucositis?: These don't sound like symptoms of sinusitis. Maybe viral pharyngitis. But you may be on the right track, i.e. oral mucositis (inflammation of mucous membranes of the mouth), a well known side effect of many chemotherapeutic drugs, including methotrexate. Discuss with the doctor who prescribed it. http://www.ncbi.nlm.nih.gov/pubmed/23601229