What could be causing the pain that goes from my hip down the back of my leg to my ankle? It is just a streak of pain down the back of my leg.

Sounds like Sciatica, a condition caused by a pinched Sciatic nerve which branches from your lower back through your hips and buttocks and down each leg. Orthopedic doctor should be able to pinpoint by doing some thumb pressure tests .

Not sure if you are male or female but if you are male, there is one thing I learned about sciatica nerve pain. If you have a fat wallet in your back pocket, it can cause this to happen. Move your wallet into your front pocket. It sure helped me.

Wallet, yes. Also whether or not you carry a child on one hip a lot, or a big heavy purse/bag.

Also how you sit at a computer! If you lean even a little to one side (usually away from the side of your handeness), you can get sciatica and other nerve pain too. Leaning forward to see the monitor sways out your stomach, curves your back, and causes all sorts of pain.
DESK must be at the height of your relaxed arm with the forearm straight out, shoulder DOWN. Lifting the shoulder even an inch to type throws your whole body off to the other side.
CHAIR must be adjustable, with the seat set at the height of your bent legs when feet are flat on the floor and there is a tiny bit of space under your thighs (unless your legs are too fat to have space under them).
BACK must be straight! This means thinking about it constantly.

I would ask around for a GOOD chiropractor.

Meanwhile, a big heating pad is your best friend. When you can, lie on it flat out, and do tiny leg lifts if they don't hurt. And one leg over the other, and bend knees up, and so on, to free the taut muscles around the sore nerve.

I have to sit on a heating pad even on 90 degree days....

There's no cure that isn't 90% your work at home.