Oh no, not another limper! I just went through a similar situation with Duncan...the intermittent limping with no apparent injury...I was worried too.
Pam mentioned Lyme disease as this is one of the symptoms and she went through it with her Bella...I have a thread under Duncan is Limping and under Update on Duncan's Limp in Dog Health if you want to check there to see what was said...I see you are in Michigan...I don't know if Lyme is common there? It is completely treatable if it is so don't panic like I did! There is a test your vet can do and get results during your visit and if it is, he can be treated immediately.
Good luck and keep us updated!