Hi Phil
You're an energetic guy, and really look after yourself, however, you do mention that you lack eating Fruit n veg (naughty lad)

... although you take multi-vits, they don't always cover everthing, and especially as you are energetic, do weights and keep fit, you need to eat fruit n veg ... banana's are especially good for you, they have potassium in them, which feeds your blood cells, and it's a good sourse of energy (better than an energy bar) and it's one of the recomended '5 a day'.
In your subject title - you mention 'mood swings' ... if you were a girl, I could answer that one

but as you are young guy, it's not that.

I think it may be the lack of nutrition - you need a balanced diet, eat plenty of fruit n veg, try it for a while, and see if you improve.
Good luck, and keep in touch if you need us to support you, or if you have any questions.
Ginger