Behavior problem with my chihuahua..?

I have a 1 and a half year old chi...and she is very sweet and loving to me, my husband, daughter and our other 3 dogs....UNLESS she is under a blanket!! For some reason, when she is under a blanket, if the other dogs get near her...she will do this viscious little growly snappy thing...but when she looks out of the covers, it's almost like she gets up and goes to apologize to her victim. I've started taking her and sitting her down on the ground and telling her "no" when she does this..but it is not helping. Can anyone tell me a good trick to get this to stop and why is only when she's under covers?? Alot of the times, she's on my lap with a blanket over her...and I'm afraid she may bite me by accident.