Leg cramps at 28 weeks are pretty normal, honestly, your body's carrying a lot of extra weight now and your muscles are just getting tired from it all. A friend who was pregnant around the same time said her doctor told her it's mostly down to muscle fatigue, and it tends to ease a bit once you shift position or stretch. The cramps in your lower arms are less common, but still within the normal range of pregnancy aches, your whole body's working harder right now. That said, it's definitely worth mentioning to your own doctor or midwife at your next visit, especially if they're getting worse or keeping you up at night, just to rule out anything else and get some reassurance.
Aiyoh yes this is very common in pregnancy especially in third trimester, don't worry too much! It's usually because of low magnesium or calcium levels, so try to drink more water, eat more banana and dairy foods, and maybe ask your gynae if you can take magnesium supplement because it really helped me a lot when I had the same problem. But if the cramps are very severe, one sided, or got swelling and redness then please go see your gynae faster faster because want to rule out things like DVT which can be dangerous during pregnancy!