I tried removing milk (I was eating milk with cereals in the morning) - which appeared to improve the situation (becoming less frequent) - but it turned out it worked only because I got rid of eating cereals in the morning and I can drink milk just fine.
The doctor said nothing was wrong and that it was just stress and other BS. They told me my cholesterol was high and told me to avoid meat (I was basically on a pasta, meat and vegetables diet) and prescribed me statins to drop cholesterol.
I ignored the advice and switched to a only meat diet; diarrhoea and blood disappeared completely in around 3 days and I started feeling way better. A lot of the small pains disappeared as well.
The only cons I've experienced is that I have less energy for workouts (during the day I feel fine) and I switched from doing sets of 5 reps to set of 3. I should be eating roughly the same amount of calories but no carbs means no immediate energy to use. I'll be on this diet until I drop a few more kgs and then I'll try to up the calories (still only meat) and see if I can go back to 5 reps. If not I'll consider reintroducing some carbs before workouts (from fruit / vegetables, in moderation).