Left arm normal in the mornings, slightly thicker in the evenings. After a hot shower it also dilates about 0, 5cm. Shoulder veins visible. Problem?

Maybe. See your dr., as it could be due to compression of the blood vessels to that arm up high in the chest--can be by bone from birth defect / trauma / arthritis, or vascular disease, lung disease, or a growth compressing the vessels. See what your dr. Says and if you need imaging to better sort out (ultrasound / ct / mri). Good luck!