Losing mindfulness and peace after three weeks of successful practise

I've been practising mindfulness meditation for about three weeks now. I started with ten minute sessions but soon increased it to a half hour and then either a full hour or two haf hours with a five minute break. I felt that my practise was making me more peaceful, less angry, calmer and generally happier. I would achieve what access concentration quite easily and yesterday I felt an experience that I found rather similar to the descriptions of the Buddhist first jhana. Immediately after that experience, it seemed to me like my mind had almost, and very suddenly, let go of the progress that I had made and I suddenly returned to my former angry restless self. I am again suddenly consumed by angry thoughts about a trivial event that happened many months ago. I don't feel the peace that meditation used to bring anymore and today I really struggled even to finish a half hour sitting. I don't seem to be able to release distractions or thoughts anymore and can't stay with my object of meditation. What's going on? What do I have to do to restore my mindfulness?