Do you have any breathing-related symptom ? Does the pain increase on coughing ? What is your diet since last 3 days ? Do you get any problem/pain in swallowing food, particularly solid food ? Do you think you are getting stomach acidity or too much gas in stomach ?
In my opinion, your symptoms are less likely due to swallowed plastic pieace. Reason, the plastic piece if at all eaten must be very small in size else you could have noticed it while chewing in mouth or sharp pain
while swallowing that particular bite. Also, as the food keeps moving away from mouth in all regions of stomach & intestines, it is unlikely that plastic piece will remain sticking in stomach in same area for 3 days. Small piece generally pass out on its own.
Still, I suggest you see your local doctor since you mentioned that it's strong pain. Your doctor can do physical examination & assess where is the pain coming from. It can be routine stomach acid related OR muscular OR just psychological. Meanwhile, you can take over-the-counter antacid
Hope I find the above information helpful. Please let me know your pain status in coming days.