Brother,
What you have tapped into here is something that has always greatly intriqued me in my walk with God. I have much to say about this sort of thing, but I'll try to crystallize my convictions in a concise way for everyone here.
Normally, I don't fast unless I'm deeply and unmistakenly impressed by God to do so. In the past, however, I would endeavor to fast frequently, regardless of my inward convictions, as I equated fasting to power in preaching and prevailing prayer. Consequently, what started with good intentions soon became a work of the flesh, a routine,
[i]a ritual[/i] of abstainment on the outside of a washed cup. Inside the cup, however, was the old, bittersweet tea that my heart still drank in secret with Adam. The fasting camouflaged it nicely; and it even had me convinced that the subdual of the flesh was wrought and held effective by external means: i.e. fasting, praying, reading the scriptures, etc.
Well, brother, God has His means of teaching grace, and His ways and chastening are such that they are designed to destroy any confidence we may have in the flesh, where all hope in and of ourselves is completely demolished, where we are rendered helpless and abased and at last come to the final and absolute place of surrender at the feet of Jesus Christ abiding within by faith in [i]His life being lived through us[/i].
Now, dear brother, if your immediate impluse in the light of sudden trials and fights following a period of fasting is to [i]continue[/i] fasting or pray harder or to [i]do[/i] something additional, you might be missing God's intended purpose in allowing this affliction to beset you. It can be a mistake to think all this happened to you because you've fasted, or that Satan is out to get you now that you've stopped and grown weaker (I know it can be tempting and self-gratifying to think this); but the truth is that these fights and trials and temptations after a time of victory can come from God himself as a chastening rod to divinely instruct grace and amazingly recalibrate your spiritual priorities.
It has been my experience that God will allow things like this to happen to cripple your trust in anything but Christ, even in spite of all your spiritual conquests up to that point. God's sole aim for you is that daily victory and joy be wrought by the faith of the Son of God living in you and nothing else; not by fasting, praying, meditation, memorizing scripture...nothing at all but Christ alone. These good things all have their purpose in the walk of the saint, but they must be kept in their proper perspective with Christ the Sun in the center, radiating out into and giving life to all them. God will often take these spiritual things and show you the futility of trusting in them for victory and peace and joy if your eye has been diverted to them from [i]Christ in you[/i]. It is a most painful, yet most loving experience.
How demoralizing it is to be elated in a prolonged fast, to have divine liberty in prayer, to be given deep revelation in certain scripture, to enjoy precious, initmate fellowship with the Lord Jesus for a season..and then to be suddenly crushed and abased by a random failure! Before the affliction, you believed the victory (or peace or joy) had been wrought by virtue of the fast, by virtue of the prayer, by virtue of reading the Bible (and could be maintained as long these activities are continued), but God sees what your heart is trusting in, and sends defeat [i]in spite[/i] of the chariots and horses you've made of His divine things. Do trust me when I say a believer can make an iron chariot of prayer, and a horse of a fast. Prayer and fasts are divine weaponry, but only when the trust is placed in the True Strength and Divine Author of such. Otherwise they are reduced to a work of the flesh, and God will soon make sure all confidence placed in them will be irrevocably smashed to the glory of the central position his Son must have in your life and to your ultimate good.
