I have a stretch mark on my thigh and last night part of it started to itch and now it's red. is that normal?

No, could be rash: The most common causes of itch on the skin are dryness, irritation or eczema. First try using dry skin lotion several times a day. Skin over stretch marks is more fragile and may be irritated more easily.

Yes: Stretch marks do itch. They also look red initially, but turn lighter color over weeks or months. Moisturizing lotion, creams or ointments will help with the itching.