Depending on your metabolism and person physiology, different strains will affect you in different ways. Some feel that kava wears off after only a couple hours, while some (like me) feel a sort of "shield" as Vekta described for days after. That being said, the day after is usually much more mellow than it would be without the kava the previous day (as long as it wasn't tudei/ISA/Koniak/etc) which leaves me very groggy the next day). I don't feel impaired, just slightly mellowed out.
If Melo Melo didn't do it, I would venture to guess you are just sensitive to the double-bonded lactones (DHM and DHK) which I believe are commonly in higher percentages in Fijian kava.
Really, only the two-day kavas (ISA/tudei/Koniak/etc) leave me with any negative after-effect the next day. Almost all kava (absent extremely heady strains) will have the "shield" effect the next day, and sometimes two but it's very subtle and not really what you described. I just feel relaxed, contentment, ease of acceptance, etc. the day after, not cognitively impaired at all though.
