Hamstring strain

i am new to this site but on browsing through it it appears very detailed and helpful.
I have a problem with hamstring strain.i had been training for 6 weeks all going well then one night lately I felt strain down at back of my knee on the outer part of leg.A sports mass age person worked once on this and told me that there was some scar tissue there form old hamstring injury .After weeks rest it felt ok and I returned to training but at the end of the session after few sprints the localised hardness and strain returned.I have pulled hamstrings in the past and this is not what it feels like this time .Is it the scar tissue that is causing the problem and is mass age the best thing for this??