If he has a cold, it is possible that the fluid in his ears is upsetting his balance. You might want to take him to the doctor to see if he has a possible ear infection.
Otherwise, it could just be that he is very tired and you don't realize it. Sometimes my kids don't look tired, but there are smaller things that I don't notice that should clue me in to their fatigue. Normally babies who are walking well one day can be so worn out with activity that they want to keep going because it is still exciting, but get frustrated with themselves. After they have been walking well, babies can usually get back up when they fall down no problem and won't cry. I don't think this is any reason for concern. Make sure he is getting adequate rest.
He could also be going through the clinging stage and want to be held a little more. I found that once my babies start to walk on their own and gain a lot of independence that way, they realize they are getting a little farther away from mom and need that extra cuddling time.
