Hello,
From the symptoms it can be due to dermatitis or stretch marks. Stretch marks occurs when the dermis (2nd layer of the skin) are frequently or continuously stretched causing tearing of the skin and lead to scarring. These marks happen commonly during pregnancy, weight gain, muscle building, on areas of the upper arms, breasts, abdomen, lower back, hips and thighs. They are initially red but then may change color to listening white.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.