Ellyjay, using heat on your thigh is fine. Just not in your knee area. Be sure and ice that knee as much as you can. Even if it's not swelling where you can see it, it can have some swelling in the knee compartments. They are so tight, it doesn't take much to give you pain. You'll find sleeping more comfortable if you ice all night too....both front and back of your knee.
I'm assuming you're taking a full dose of your pain meds before bed. But....if you are uncomfortable, you need to take another full dose 4-6 hours later. Set an alarm to wake up if you need to. You are still early in this recovery process and taking meds all around the clock is perfectly fine. It's important to keep the pain managed.