This applies to anything in Catholic doctrine.
If you are not fully aware and have not given your full consent consciously, knowingly and soberly, that thing is not binding. In other words, you have the ability to say, 'Hey, I didn't know what I was doing and I take it back'.
Now, God does not like people making solemn promises to Him and later taking it back , as we see in the Old Testament (which is why I try to limit what I promise God, and instead ask Him for the grace necessary to do a thing which is for His glory).
Your case is slightly different, though. Your intentions were to pledge something to God (through Our Lady), and had a good intention to offer your sufferings for a greater good (of the suffering of other people). That said, it would be wise to only 'defer' graces which don't include those garnered by your headaches or anything you pledged. If it was a light-hearted pledge but with a good intention, I can't see any reason why Our Lady or Our Lord wouldn't allow you to take the pledge again in order to set things straight, so to speak. On the other hand, if you really knew what the pledge was and fully consented, then taking it back now ( or wanting to take back the ability to use those same graces for other things also) kind of defies the purpose of a pledge altogether,as I'm sure you'll agree.
God is the most rational person I know ;) He'll know and understand. I wouldn't lose sleep over it, as God knows your intention is good, and your question's very existence is physical proof of that.
My advice would be a solemn prayer to Our Lord or Our Lady and whoever you pledged to asking that you amend your pledge to include a limited set of your sufferings as offerings, so, as you desire, you can defer your graces as you please. Personally, I might start asking God to use His graces as HE pleases, isn't that the perfect way to use them? Still working on getting off my high horse to hand Him the reigns over, though.