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Tar stool is a sure sign for GI bleeding, it would be lightheaded not so see a doctor. You should at once if you have upper belly pain. Please stop Ibuprofene at once, this is most probably the reason for your gastritis or ulcer, and don't drink any more coffee. Beer and strazonone is not a reciommended mixture but not responsible for tar stool (except the gas in the beer).
Maybe you can keep bills down by making a stool test to find out if it really is tat stool and a physical, take antacids like omeprazole then for three to sex weeks and see if you again have problems. But actually you should have a gastroscopy to be on the safe side because the suspected bleeding can increase anytime.
I wish you all the best!
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