Help--my baby has started crying and cannot stop!

Ok, Moms, I need some guidance. This is our first baby and she has been an absolute dream until very recently. She is just a little over 6 months old. For the past week and a half or so, she is just about inconsolable. She cries if she is not sitting on your lap or being held. I cannot even sit on the floor with her and try to get her to play for more than a few seconds before she starts crying to be held again. She is also not sleeping at night anymore. She wakes up about once an hour just screaming. I can usually console her and get her back to sleep pretty easily (thank goodness), but the daytime is just miserable for her right now. She just doesn't want to do any of the things she used to love to do, like play in her exersaucer, tummy time, swing, etc. She is absolutely panic stricken and hysterical if you leave the room. And once she starts crying to be picked up, there is no turning back. I have tried to let her cry for a few minutes to see if she can pull herself out of it, but she is so worked up, she gets choked and has a coughing fit.She has been to the pedi and nothing wrong, no ear infection or anything like that they said. She has been teething but does not seem like she is now (I mean, she is not chewing as much as she when she got her first tooth or drooling as much). I just wish I knew what to do for her to be happy again. She gets this panic stricken look on her face when you set her down and just starts wailing. Is this a phase, is there something I am doing/not doing? Any physical things I might not be looking for? Any advice at all would be greatly appreciated. I feel like I am failing her now and I don't want her to pick up on my insecurities also.