It's my initiative, and I cast Time Stop. I get 4 apparent rounds to act within the limits of the spell. Note that in "real time", all the following actions effectively happen within the standard action of casting Time Stop (or swift action if you're somehow casting it quickened).
Apparent Round 1: I cast Delayed Blast Fireball on my Ice Giant enemies, which works since it would not take affect during the Time Stop. I then move 30' toward the one that has barbarian class levels.
A. Round 2: I move in another 30' to where I am right in front of him and Gate in a Balor.
A. Round 3: I Dimension Door back to safety.
A. Round 4: I cast Stoneskin on myself just in case.
Then the spell ends, and I get a move action (if I did not take one for the real round). Everyone else acts as normal, and my DBF takes affect whenever it was set to take affect.
Personally, I wouldn't say that you could cast Time Stop within a Time Stop, since it would have to rely on apparent apparent time. Plus it'd just be a twee broken.