Mr Left, when stretching the structures at the back of your legs , it is as well to remember there are more of them than just muscle , in fact a forward bend with legs straight will stretch nerves, tendons, muscles, vessels , fascia, capsules, fibrous material , joint tissues, skin as well as put a certain amount of torque and tension over bone and their attendant local soft tissues as well.
Any attempt then to "feel" that it is muscle being stretched , will be masked by the myriad other sensations absorbed by and interpreted by your brain.
In most cases, it will be the sheath surrounding your nerves(dura) that will provide much of the sensation of stretch. Second on the first feelings list willl be those tissues with most free nerve endings and receptors for stretch, these are the ligaments and attachments to bone at the ends of muscles. You may never "feel" muscles stretch at all , in most stretch situations it is the first two that overwhelm your sensory perceptions, rendering somewhat neutral any arising from muscle itself. Muscle has very few stretch receptors and free nerve endings compared with ligament and dura.