Hard steering and low tire indicator light

The other day I was driving and noticed my steering seemed a little harder. Then, my low tire light came on. Stopped at Walmart where I got new tires 3 weeks ago (probably a mistake). They checked tire pressure, said it was ok. Reset tire light and added power steering fluid, it was a little low. Still have hard steering. Any ideas what this could be? What else should I check? It seems more than a coincidence that my steering problem and low tire light came on at same time. Possible problem with tires, unbalanced?