What causes body and muscles aches after a hot bath? every time i have one a few hours later i crash, bad fatigue, body aches and nerve pain. what could it be? being investigated for autoimmune disease as ana 1:320 and nailfold capillaries dilated?

ANA 1:320, especially with homogenous pattern, is significant but non specific. People with lupus and fibromyalgia are intolerant to heat but usually heat from the sun, not from the hot bath but I cannot rule it out in your case. You should avoid hot bath because it may trigger the flare-up. Continue the investigation, DsDNA & Smith Ab are + in lupus (skin or systemic) but not in fibromyalgia.