What Causes Stiffness In Neck And Shoulders After Exercising?

Hi, I've recently started exercising everyday for the past 3 weeks now, just trying to lose some weight, I'm quite overweight and have been trying to slim down. Recently (For a couple days now) I've been feeling light-headed (Especially after eating) And have had a stiff/sore neck and sore shoulders, just curious whether this could be a more underlying problem, or... what I should do, I don't know. Any feedback would be appreciated.