Poszłam do sklepu. - I went to the store (note, without other qualifiers this sentence may not be informative enough, eg. we can't know if you came back from the store)
That's interesting, because English makes a distinction here. "I went to the store" means you went there and came back, and "I have gone to the store" means that's where you've gone and you're still there. "Poszłam" can be interpreted both ways in Polish.
pam, I really can't see why anyone would say you were wrong for using "poszłam," because it should be clear from context which meaning you intend. "Byłam w sklepie," I guess, is a periphrastic way of getting the there-and-back meaning unambiguously, but it's just another way of getting the same meaning across. In English you could say "I've been to the store" to pretty much the same effect. Which form you use is really up to personal preference.
Maybe your friend corrected you in a very specific context, or maybe they have a very strong personal preference for a particular form to the point where alternatives sound unnatural to them. Without further context, I can't even suggest that your friend might be a grammar nazi, since "poszłam do sklepu" is perfectly-formed, grammatically correct Polish. The only situation I can imagine where you might be wrong for using "poszłam" is, as gumishu suggested, if you drove to the store, since "poszłam" has an implicit meaning of "I went by foot".
Regardless, "poszłam do sklepu" can express both "I went to the store" and "I have gone to the store," and it implies neither that you're still at the store nor that you're no longer there. It is not necessary to add anything else, because it just says that at some point you went to the store, and whether you're still there or not is left up to context. By all means, you can avoid using "poszłam" to keep your friend happy, but I don't think you need to extrapolate your friend's suggestions into an all-encompassing grammatical rule.
This is why for some things, i can't rely on getting correct information from my friends.
I think this might be the case here as well, although I can't say with certainty without the specific context in which you were corrected.
When i;ve spoken to her, we've compared Polish and Russian words.....they really are incredibly similar.
I stumbled on a subtitled Polish translation of a Russian pop song a few days ago,
youtube.com/watch?v=jnCZ0lqBoAM
and, tbh, the words are so similar in the two languages, that I can't see the point of translating it. A glossary of maybe a sum total of five or so words that were significantly different would have sufficed. That said, language more advanced than that used in a commercial pop song can be significantly different, but still not drastically so. The languages definitely seem far more similar than English is to its closest relatives, Flemish, Dutch and Afrikaans.