Nikos, if you are eating antelopes for bfast that is surely causing your digestive problems... I think you meant
cantelopes! Sorry, just giving you a hard time!
When you transition to eating a healthier diet lots of things will happen inside your body. You definately will go through an adjustment period. If you are adding more fiber to your diet you will likely need to use the bathroom a bit more. Stomach problems can occur as you wean yourself off of high fat or high processed carbs. Or you may just have a sensitivity to one of the fruits in the fruit cup.
Its possible that the fruit cup you are eating has too much sugar for you - is it packed in heavy syrup? Its best to eat fruit that is all natural (in its own juices) or whole fruit. Also, you may respond differently to different tyrpes of fruit. Think about trying a whole apple or whole orange or banana and see if it makes a difference. You may need to play around with different menus to come up with the best meal for your body.
You mentioned your previous breakfast used to be english muffin, eggs, and sausage. You know that is NOT terrible - I would recommend that you skip the sausage and instead have english muffin, eggs, and fruit. That will help keep the fat down, you will still get adequate protein and fiber.