Baby crying unless held

Linds

I have a sixth month, and she cries all day long unless I am holding her and standing. She will play in her bouncy seat, excersaucer, etc for like 4 or 5 minutes max and shes done. When shes tired, I cant rock her, bounce her, or lay her down. She just screams and screams. Its not like colic.. just like she is fighting sleep. I can handle that, but I have a 4 yr. old, and I babysit a 3 month old as well, so it's super hard to hold her constantly.. any suggestions??