stock floor ranger problems...

my ranger is stock floored with 3x3 from the fire wall to behind the cab... infront the firewall is a toyota front clip... behind the cab it ties in with 2x3 tubing going up at an angle then goes straight back when it gets to the axle. i got everything together bagged it up body dropped the bed and everything... i put the bed on... and the frame seems to have a lil slack in it...when i start airing up the rear of the truck i notice the bed lifts itself up a little bit befoer the while truck starts lifting. i cant figure out why this is....this truck was already done once before and i cut everything off behind the cab and rebackhalfed it... could the 3x3 tubbing under the cab be tired and flexing? am i missing something here? i dont know where the problem could be...if you check out the pictures of my frame you will notice that there is a notch in in for the axle.. i know for a fact thats not what is flexing because the bags are mounted before it..let me know what you think.. i am stressin big time right now... i been working on this thing for 10 months and i get down to the last bit and this happens.