For example, your quadriceps muscles are a group of four that are located at the front of the thigh; one of the group members, the rectus femoris flexes the hip, which brings your lower extremity (thigh, lower leg, and foot) forward, in front of you. On the other hand, your hamstring muscles are located at the back of the thigh. When they contract, they extend the lower extremity, bringing it behind you.
Some of these red flags are much less red than others, especially depending on the circumstances. For instance, “weight loss” is common and often the sign of successful diet! (Well, at least temporarily successful, anyway. 😃) Obviously, if you know of a harmless reason why you have a red flag symptom, it isn’t really a red flag (duh!). But every single actual red flag — in combination with severe low back pain that’s been going on for several weeks — is definitely a good reason to get yourself checked out.