Omg! What i do now
Well, you can safely assume that you are not the first person to swallow a watermelon seed (I do it all the time). If you don't know of anybody that has died due to watermelon explosion and you've never seen any news reports about it happening, you can probably rest assured that it won't happen to you. Plants produce fruit because animals that eat it spread the seeds they contain when they defecate. Seeds will not germinate inside your body. Even if they did, and they survived the extremely acidic conditions of your stomach, plants grow by photosynthesis, which requires sunlight and there is no light inside your body. If by some stroke of luck you have swallowed a lamp that produces photosynthetically active radiation and a seed that can grow in acid, both of which don't move through your digestive system like everything else you swallow for some reason, you'd need to swallow some very special bees or something else to pollinate the flowers before a fruit is formed. I tell you what, see a doctor if you develop a stomach ache that persists for a couple of days, and please share medical imagery of any internal gardens that develop.
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@shereeyaa The link @Curmudgeon has put explains it all.