All tires are slowly losing air. What matters is how you define "slowly".

Slow. I filled them up last weekend to 90 PSI. Today they were down to 45psi.

Did you buy the bike from the LBS? If so, this tune-up (and several afterwards) should come for free. If not, it's a good idea to have them look over your bike and fix whatever problems it might have...
No, it was a Craigslist bike.