If I don't bleed after every bm but only when I eat beef which makes me constipated, could I be bleeding from straining?

Time for an exam. At your age bleeding from hemorrhoids or from simple straining is not expected. I would wonder about something else. Perhaps a fissure? In any even an exam is indicated and then a plan to eliminate constipation as an ongoing issue. It would be unusual if beef was the sole reason for the constipation.