Muscle spasms are certainly a side symptom of OA and RA for a variety of reasons. A nutritional imbalance could be the culprit, or dehydration. Also, when your neck hurts, for example, your muscles unconsciously tense up to "protect" the sore area. The constant stress causes the muscles to become tired, sore and actually inflamed. Cramping, spasms and nerve compression can result.
The pain seems to spread throughout your body because, like the song says, everything in your body is connected to everything else. Back pain makes you unconsciously alter the way you walk, stand, sit, even lie down. Back pain can cause your hip to hurt, which in turn causes knee stress, and on down the line until you seem to hurt clear down to your toes.
If you aren't already performing a series of gentle exercises to strengthen and stretch your back muscles, ask your doctor for a list or a referral to a physical therapist. The exercise will strengthen your core and the supporting structures around your spine, which will help with the pain. Weak muscles in your back and abdomen = more back pain.
Hang in there!
It could be from muscle atrophy or could be fibromyalgia (often happens in conjunction with many types of arthritis) or something else. It is important you see a rheumatologist for a proper diagnosis and treatment.