I have been out of the gym for about 6 months because my wife has been pregnant!  i have been back at the gym for a couple weeks now and on back day twice now i have come home with about 4-5 knots in my upper back to the point where it even hurts when taking in a deep breath! 
The main exercises i have been doing are deadlifts, pullups, bent over rows, and cable rows.  
Would this be caused by bad form or because of being out of the gym for a while?  I am not really going heavy at all and just trying to work my way back to my old self!   
thanks!