I know how you feel! I let my little sister take my ds for a walk when he was only a week old - because I was getting totally harassed about being "overprotective" by my family. Grrr. Thank God Almighty, he slept the whole time, but I was so mad at myself for doing that that I made a rule that we wouldn't be separated again unless I was totally comfortable. I don't think I left ds to go to the store (leaving him with dh) until he was 8 or 9 months old. I would go places that dh could walk to, in case of emergency, though. This might sound extreme to some, but I don't think babies were meant to not have access to their mamas - so even now at 13 mo. I always make sure I'm never more than walking distance away from him (about 2-3 miles tops.)
This way I don't have to worry so much about him crying the way you describe. There have been a *couple* of times that I made bad decisions and let dh take him on a bike or in a car for a couple of miles, and I have been mad at myself when that has resulted in him crying really hard (shudder). DH always takes the cell, and twice I've had to run out to meet them. . . It really is *so* hard to know sometimes! I like to err on the side of caution because I don't want to traumatize ds or deal with the guilt, either!
MisfitMama