I started using Clindamycin/Tretinoin about a month ago. My face got really clear for the first time in my life about two weeks ago! I never experienced an "initial breakout" until now -- it was just constant improvement. 
I never noticed any irritation either, and only one spot of dryness.
Now, my face is BURNING constantly, especially after I apply the Ziana (Clindamycin/Tretinoin 0.025%) gel.
My acne prior to the gel was pretty constant acne on my cheeks, with worse acne on my left side. Right now my entire left side is broken out fairly badly, extending to my forehead even. My right side is taking a beating as well.
I have never had severe, or cystic acne. My acne is mild to moderate, right now more on the "moderate" side.
So what is going on? Is this normal? Is my "initial breakout" just delayed? Sadface. 
I recently tried Ziana - called Biacna in Canada where I am from. A little history - I took Accutane about 10 years ago and had great results. However, I could only maintain by staying on the pill. I went off it once to have a baby - got pregnant right away, had great pregnant skin, and back on the pill after. Then we decided to consider another kid. I went off the pill, but this time we did not get pregnant right away and my acne slowly started coming back. Not as bad as before, but I was starting to get cysts that hurt and it was awful. I went to the derm who suggested that I try a topical before committing to going back on the pill forever or trying another course of Accutane. So she prescribed the clindamycin/tretinoin gel.
Just like you I had a great first couple weeks and was feeling very positive. Then, all of a sudden I was getting huge bumps under the skin all over my chin and my acne, within a week, was worse than ever. I hoped it was an initial breakout and kept going for several more weeks. The derm had said to try it for 12 weeks before making a decision. Around week 8 I couldn't take it anymore - it was so itchy and painful - and I really thought that it should have been getting better by then. I called my derm and went back. She agreed that this should not be happening, so I stopped using it. In the end I went back on the pill, and, because the Ziana made my skin so awful I am now taking another course of Accutane to clear up the damage.
I have read so many positive reviews of Ziana, and so many medical journals that showed great improvement for people taking it. I really believe it is helpful for most people, but I also know that it was very bad for me. I would suggest that you see your derm to see if your reaction is normal, because mine could tell right away that something was not right. They have a much better idea of what is 'normal' and what results to expect as you go than we do. They would be able to tell you if what you are experiencing is an initial breakout, or a bad reaction to Ziana like I had. I wish I would have gone in when my reaction first started happening and maybe I wouldn't be back on Accutane now. Alas. Good luck to you, I totally feel your pain.