Calf pain

Hi I'm marathon training, been running just over a year had no probs with calf pain before, done a couple of halves & a full last year. After last wks long run felt fine, rest day next had a dull ache in both thought I'd over done it previous day. Next day intervals a little un comfatable, next run was 10k took it steady a little discomfort. 2 rest days before long run, felt ok in morning went out did run, no probs or pain, or next day. Next day from no where a little ache & carried on thru hill sesh got slightly worse during. Next day quote sore. Can't decide if its a strain or just aches. Hasn't happened before now. Just doesn't feel right. Any thoughts or advice would b gratefully recieved