I am not a man; however, have you changed anything? Added work outs? meds? Supplements? Masturbated less or more? Maybe you have a great deal of testosterone hitting you? And that combined with emotions can, well, make a change.
Try masturbating frequently or before you see her & see if that helps. Look under the thread of "squeezing techniques"...
http://www.sexinfo101.com/forum/sexu...ulation-2.html
Perhaps the degree of excitement & stimulation is too much for you. I know some younger men have to change their focus to "things" [much like what you described] to last longer.
Additionally, try focusing on her alone [her enjoyment] & not on what you are feeling/enjoying until you can postpone an orgasm. I have only found this with younger men; also stopping and performing oral on her for a bit and then returning to intercourse may help.
Or work out multiple sessions...