You might want to mention it to your pediatrician at his next appt, however I have read and was told by our pediatrician that this is normal from time to time in infants. My doctor even asked me if my daughter was doing it and he said if she did, it was normal. She wasn't doing it at the time but she did later - it was reassuring that I was already aware that it is nothing to worry about.
Again, I think it is normal, however mention it to your child's doctor at his next check up - he/she will be able to reassure you.
I would ask your pediatrician for a referral to a pediatric ophthalmologist as this is not normal.
Sometimes an eye of a baby will looked crossed but isn't and its due to fold of skin at inner corner of eye that adults don't have this is called "pseudo-strabismus" or false crossing. Best let the pediatric Eye MD make the determination.
Remember that strabismus affects one in 50 children and is a major cause of poor vision in one eye (amblyopia).
JCH III MD