Susan, one thing to check (or better, have a shop check) - the alignment of the derailleur hanger. The hanger is metal tab that hangs down below and behind the rear wheel axle on the frame. The derailleur bolts onto it. The hanger can either be an integral part of the frame or a separate bolt-on piece.

Those hangers are very commonly bent when rear derailleurs suffer even small damage. A bent hanger can lead to poor shifting and even the derailleur hitting the wheel spokes. A derailleur getting wrapped around by the wheel will get expensive (sometimes very expensive) quite quickly and you won't be riding home.

A home mechanic can do the check with improvised tools but some knowledge and care is required. Shop have the tool that makes it child's play. I'd guess a price from $25-40 from most shops.

Remember the above if your bike ever falls over to the derailleur side, even if you are not on it. And even more important - be kind to your toe and heal fast!