This sounds like something to discuss with your doctor.
The easiest - not-so-well educated answer I can give is that eating stimulates the vagus nerve. The vagus nerve origins in the brain, but it has motor functions in the stomach.
When we eat, the body focuses on the digestive system, which might cause our bloodpressure to drop a little bit. Heavy meals are tougher for the body to tackle.
If it is a recurring problem, you should discuss this with your health practitioner as this forum is no diagnostic tool and offers only communication between its members and can only provide general information.
ParamedFlorena