Definitely don't fret.
Definitely get checked out for anaemia.
A week really (really, that is) off from running should give minor bugs the chance to buzz off, or else make themselves known.
If you've been reading through training threads, you'll remember that there isn't a person on them who hasn't the experience of going sluggish like this. A lot of the experienced ones even seem to expect it, so at least it doesn't upset them when it happens.