Combat Corpsman 75

I just wanted to see if anybody out there has been short changed by the VA IRT retrograde payments. I am 100% service connected. I filed 01 March 2015 and received my first VA disability check on 01 July2015. I received a "retrograde payment" on 11 June 2015 in the amount of $2,296, and just received $4,240 on 25 August 2015. I then received a letter form the VA stating that my only compensatable months were April and May. does this sound right to anybody? I feel like I'm being shorted 2 months of retro payments. Any help would be great.