Lombago. Perhaps the problem is solved thanks to 3 days of rest and strong anti-inflammatory pills. The thermal baths helped too. However the doctor said that the cause was the swimming: too many metres of free-style and too "fast". Now I have to follow a different program including more relaxing works. It means 15 days far from the pool and running every other day until august 1st.
Happy running and enjoy this wonderful summertime.