This is may help:
Place your thumb pointing downwards on your cheek just outside your ear where your ear canal meets your cheek. Support the other side of your head with your other hand.
Apply some pressure and slide your thumb towards the midpoint between the front and back of your jawbone / jawline. As you pass your molars will find a fleshy bump, it is your masseter muscle, it's function is to close your jaw. It runs from your cheekbone to the back part of your jaw. Use your thumb to find a tender spot look for the most tender spot and push on it for 15 seconds. You should feel a dull ache radiate out from the spot, sometimes it goes to the part of your head above your eyebrows, or lower jaw or front of your ear. Work at a pain level of between 6 to 8 on a scale of 1 to 10, 10 being unbearably painful. Repeat twice. Look for other painful points on the masseter and do the same press and hold for 15 seconds . There are 4 possible trigger points on the masseter. You may or may not have all 4. Finish off by opening your mouth running the base of you palm along the masseter which runs from you cheek bone to the back of your jawbone. Open your mouth wide (but don't over do it) and keep it open for 20 seconds to stretch.
If your muscles ache wet a small towel, it must be wet but not dripping wet, microwave it for a minute or two let it cool down to a comfortable temperature and apply it on the aching muscle. Leave it there until it is luke warm. Remove and wipe dry the skin. It serves as both a painkiller and a muscle relaxant. Repeat hot towelling if it continues to ache the next day or two. Note: use wet heat if you use a heat pad, put a wet towel around it.
Look up Dr. Janet Travell's Myofascial Trigger points if you want more information.