I have pain inside vagina,during fingering or penetration. is it serious or it could be just muscles?

Pelvic Exam: You need a pelvic exam to answer this question. See an Ob Gyn or Physician who you trust to do your pelvic exam. There is a reason why you are having this. Infection, hymen intact, lubrication. Need office visit to resolve.

Many causes: Anything from anxiety/spasm to vaginitis to herpes outbreak can cause such pain. If you have pain when you are relaxed, and while using a lubricant, get an examination by your doctor.