Definitely second the advice to go see a Physio.
IME, most muscle pulls are due to a lack of mobility, not strength. If you have been doing a lot of intense strength work and little to no mobility/flexibility work, then there is a good chance that your strength and mobility have gotten out of balance. Your hamstrings have become, for lack of a better word "brittle."
So, I would highly recommend investing some serious time doing some dedicated flexibility work (IME you need to hold Stretches for a minimum of 1 minute, 90 seconds to 2 minutes is even better, for you to gain noticeable ROM), and I would recommend adding in some mobility work into your strength training sessions.
Hope this helps. Good luck.