Is it normal for me to have fluid behind my thigh?

It depends. It depends on the location. Behind the thigh in the mid to upper thigh region would be unusual. If it is lower down near the knee then that is not uncommon and may represent a cyst or fluid filled cavity. It would be important to know whether you are talking about a swollen area (muscle strain), a discrete mass (tumor or fatty lump), or a fluctuant area (fluid filled space).