I have severe pain in my left deltoid muscle.  This has been hurting for several months now, but the pain is becoming worse.  To my knowledge, I did not injure it.  If I run my hand over the muscle it feels like there is a lump.  I have great difficulty reaching toward my back, like to unsnap my bra and reaching high over my head.  If I roll over on my left side at night, the pain is bad enough that it wakes me up.  Otherwise, I am a healthy, white female, age 55.
Hi, A lot of causes may cause that pain but in my opinion the lump you feel may be the main reason especially if it is painful and wakes you up at night, so you need to visit your physician for physical examination to know exactly what is the lump type, consistency, tenderness, bony or flesh soft or hard. For now, you may use some topical analgesics, Advil tabs after meals in addition of Acetaminophen tab. Physiotherapy is very effective in such cases it may be some tendinitis.  Hope the above information helps you. Any further clarifications, feel free to ask.