A.Z.
My daughter was diagnosed with strabismus due to being farsighted (accomodative strabismus). We first noticed it at 9 months, and took her to an eye doctor who checked her vision and recommended waiting. At 12 months, we were noticing it even more so we took her back. She was diagnosed as being quite farsighted (all kids are, but she is much more so: +4.75 is her prescription). She has worn glasses, which help immensely since 14 months. If you'd like to see pictures of my daughter with the crossed eyes, check here: http://toddlerglasses.wordpress.com/2008/03/15/how-did-th...
Strabismus can be caused by eye muscles not working correctly, or because of the vision problems (like my daughters). I would recommend making an appointment with an eye doctor that works with young children. They'll do some vision tests, and probably dilate her eyes to check the curvature of the back of her eyes. If her eye is wandering, it makes her more at risk for a lazy eye later on, since she's probably seeing double and so ignoring the input from one of her eyes. The good news is that this is pretty treatable with corrective glasses, possibly patching, and if it's a problem with the muscles, possibly surgery. Since it's happening intermittently, I would guess that it's a result of her needing to work hard to focus, so it's probably a matter of getting glasses.
Take a couple of pictures of your daughter straight on with the flash. If you notice the reflections of the flash not lining up in her eyes, or that one eye has red eye and the other doesn't, print those out and take them with you to the eye doctors. We did that and our eye doctor found it to be very helpful, since Zoe doesn't always cross her eyes.
Good luck! Please let me know if you have any questions at all about this. I don't think you are overreacting, and it's a good thing to get checked out, but know that if it is strabismus, it is not a scary thing.