I've been treated for helicobacter pylori about 8 years ago by taking antibiotics. Is it possible to be reinfected by the same bacteria?

Sure. Reinfections can occur and you may not have eradicated it by the first treatment. Get another test done and see if you need to be treated, again.
Yes, but uncommon. Rx is not 100%, i.e. Not everyone gets cured, despite a variety of treatment options. Breath test or stool test available to check for presence of current infection-both noninvasive. Need to be off ppi rx, e.g. Nexium (esomeprazole) for 2-4 weeks prior to test. Of course, may be something else bothering you. See your doc.