It sounds like something I used to get when I first started. If you don't ground and centre yourself before a spell you are draining your own energy, this makes you feel tired, lethargic and weak. If it happens again have something sweet, it should help a little but you should always try to draw energy from other sources during a spell to prevent this.
The spell could have worked, or it may not have but you definitely drained energy during the casting of the spell.
I hope this helps.