Known neurological side effects of this drug include dizziness (16%), somnolence (up to 14%), insomnia (11% to 25%), fatigue (11%), and others.
So yes your symptoms may be explained by the use of Effexor. This drug takes a minimum of two weeks to kick in; soon enough your body will no longer feel those side effects as much.
If however your symptoms persist or get worse, contact your treating physician. Also do not coadminister other drugs without reading the insert to avoid drug-drug interactions.