I'm sorry, i think you're unlikely to get an accurate diagnosis without having it assessed properly.
Is there any specific reason you haven't been to get it checked out by a professional? - (preferably a physio, but I'm biased..)
Having to take analgesia at night is generally considered a bad sign as the pain must be fairly severe. Therefore maybe worth hassle of getting referral to physio. - It may be quicker to get a referral from A+E, because you'll be fast-tracked.
As far as I'm aware, (and someone please correct me if I'm wrong!) muscular strains are generally the result of excessive eccentric (where the muscle is contracting to decelerate/resist lengthening) loading on a muscle. I believe you are more prone to developing a strain if the muscle is "cold" (like at the beginning of a workout), or fatigued (if you've pushed it too far). I also have a feeling that you are also more likely to suffer a strain if you muscle is working hard eccentrically whilst already in a lengthened (and therefore mechanically disadvantaged) position. Therefore, although your calf maybe (depending on technique) in a lengthened position during the classic hamstring stretch, it is unlikely to be having to work particularly hard in this position, and therefore unlikely to develop a strain...
Sorry can't help more, maybe someone else on here can offer more of a suggestion...
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