Calf hurts when i bend my foot up. what could be wrong?

Back of the leg: Dorsiflexion (bending up) of the foot elongates the calf muscle (aka the achilles tendon). A tight achilles tendon may be one reason this motion can hurt. Another can be a more serious condition, a DVT (deep vein thrombosis) which is a blood clot. If there is heat, swelling as well as pain with squeezing of the calf muscle, go to the E.R.