Both of my arms on my upper forearm have a lump under skin what could that be?

Sounds like. Lipomas to me, but it could be neurofibroma, schwannoma, or any number of benign subcutaneous tumors. Can't tell more from this brief descriptions. Have it checked out.
Probalbly lipomas. If these areas have been present for many years and especially if there is a family historry of similar areas, they are most likely lipomas. You should have this evaluated by your dermatologist as it could be other conditions some of which could be serious.
Simple vs serious. Our bodies always have things that grow and change. Skin, fat, muscle, lymph nodes, bones, can change shape, get larger, more prominent due to many factors. The vast majority are not cancer but simply unusual growths like fatty tumors, cysts, skin tags, moles, etc. Most cause no symptoms and can be watched. If they cause symptoms or change colors, bleed, bruise or hurt, then see your doctor.