Odd itchy kinda pain when I excersise

Don't know what it is lately but I haven't excersised for about over a month. I started a diet called Lighterlife which is a VLCD. Now before you guys bash me about how bad it is, I'm doing it knowing what the consequences 'may' be. But it's a risk I'm willing to take so please don't tell me to stop, I hear it way too much.

Using the cross trainer or elliptical trainer, my upper arms had this stingy and itchy kind of pain, it was a little painful but more irratable. Felt like as if my muscles where vibrating. But it wasn't the usual kind of vibrating motion that I normally feel when doing crunches for example when we shake a little. Same thing happened when I was running on the treadmill, had this horrible pain in my behind lol. It hurt really bad and I don't know what it is.

Does anyone know?

I am very very sad for you my dear..to see you wrecking your whole future with the way you both have & are mistreating your precious body. You are only 19 for Gods sake and yet you say in your write up 'I've been fat all my life'. You are still a TEENAGER..your life has not yet begun..and even so you are setting yourself up to fail.
Plkease throw away the lightelife foods and learn to eat a good healthy varied diet of fruits, veggies & lean proteins. Learn to eat the calories your body needs to exercise and you will end up with the 'look' and body you so desire.....the discomfort you are feeling is your body trying to tell you that you are abusing it..you will lose weight but it will be temporary.
If you can't do it with the help of those here, then PLEASE seek medical advice. I do know wha I'm talking about.

I'm on a really low calorie diet and I don't ever feel like that.

You probably need some more protein.... A lot of LCD don't advise working out because of how little you're fueling your body, so if you're going to work out you'll need to increase your calories.