So, you still have the 'stub" of your I-94 that you received when you entered the US back in Febraury? You shoul dhave ensured you surrendered that to US immigration when you crossed the US/Mexico border. the difficulty no is there is no proof that you left th ecountry within the 6 month requirement.
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Your case may be a little unusual. You may be thinking it will be easy just to come back into the US now via the Mexico/US border. IMHO, you may strike some difficulties. Since you left the US is May, document requirements for that border have changed, and passports are now required for entry back into the US (or, special DL's and other specialised documents) I would explain your case totally to the border agent, and, as another poster recommended, have some proof of the lenght of time you spent in Mexico. I do not know if US Immigration issues I-94's at the US/Mexico land border. Regardless, make sure your passport is stamped when you re-enter. Your concern at this point should not only be getting back intot eh US now, but, trying to avoid any future problems with what may, on paper, appear to be an overstay.
I am under the impression that even though it is not regular practice for Mexico officials to stamp a passport upon land entry, they will do it upon request. If you have the same situation arrise in the futuer, ensure you surrender your I-94, and get a Mexican entry stamp.
This is actually also relevant to non_US residents who are visiting Canada, and want to avoid any possible apparent overstay in the US.