You should ask some Pole on this matter, don't you think?
In fact, at the beginning of CK timeframe, Poland WAS considered as Kingdom, as Duke of Polans, Boleslaw "the Bold" was crowned in 1076 by the pope during the time of investiture conflict. After that he was expelled from kingdom for beheading bishop of Krakow, Stanislaw in 1079. After that his brother Wladyslaw Herman become duke of Polans (but not managed to be crowned) the sons and further heirs Wladyslaw Herman in 1138 partied Poland between themselves creating several duchies (that more or less are connected with division of Poland in CK) with senioral duchy of Lesser Poland (Krakow).
The idea was that the oldest of heirs would be considered as "high duke of Polad" ad except his own duchy he'd also be duke of Krakow and Lesser Poland. Of course, it was constant civil war between brothers, cousins and their heirs. During the time of feundal fragmentation Poland lost most of Silesia (the dukes of Silesia had alot of sons, and further divided the province) to HRE, both selled or conquered.
The next king of Poland was Przemyslaw of Greater Poland who become king in 1291, than was short time reign of Czech kings (Wenclaus II and Wenclaus III from 1300 upto 1320). From now on the last two of Piast dynasty were kings (Wladislaw the Elbow-High and Casimir the Great).
The kings of Piast dynasty were here until 1370. Of course the remains of house of Piast were still on Silesia and (more or less independant) Masovia (the last Masovian Piasts died in 1526, while last legitimate Silesian Piast died in 1707).
After Casimir the Great the King of Poland become Louis of Hungary (from Anjou dynasty), who was son of Casmir's sister. After him his daughter, Hedwig was crowned king of the Poland (to ensure that she ruled independetly and not as consort to anybody). She married Grand Duke of Lithuania, Jogaila in 1399 and died not so later month after ging birth to their daughter. Jogaila establish the dynasty of Jagiellons which had ruled as kings of Poland (and Grand Dukes of Lithuania) up to 1572.
To make long story shorter: for timeframe of CK Poland was ruled by King or somebody that was almost the King, or High Duke that was on King-level.
Al.
P.S. Sorry for any misspelling, grammar and any other mistakes it is 10 am now, and I am after third night without sleep.