Do I need a Polish passport to travel to Poland from the U.S.A if I hold U.S and Polish citizenship

As a Polish citizen, you cannot be denied entry to Poland, but ...

Entering Poland

1. If you use your US passport, you may be treated as a US citizen, but only as long as anyone will notice that you are a Polish citizen. Given that your name is Michal Jastrzebski, it strongly suggest a Polish heritage, and most likely therefore, a Polish citizenship (ius sanguinis). You may try entering as a US citizen, but it may result in further complications, especially on exit. It is also illegal, but as for now, there is no penalty.
2. If you have any proof that you have a Polish citizenship, you will be allowed to enter Poland, most likely after some investigation done by the Stra? Graniczna (Border Guard). This may be a Polish birth certificate, expired Polish passport or Polish national ID. This will allow you to enter Poland, but check Exiting Poland for further information.
3. If it is possible, the simplest option is to request a Polish passport from the Polish Embassy in the USA. You will arrive in Poland will full documentation and everything will run smoothly. This is the most recommended option.

Exiting Poland

That's the tricky one, I guess. If you try to leave Poland using a US passport, you may be denied boarding the plane by Stra? Graniczna. The reason for that is that Polish citizens are required by law to use Polish passport (or ID, where suitable, not in this case) when leaving Poland. Well, you might think that in that case you will use your US passport and pretend not to be a Polish citizen. It might not work, not only because your name reveals your ancestry, but also you might be recorded as a Polish citizen, for example if you have entered Poland using tip no. 2. This will result in great hassle (escorting from the terminal by security, maybe a fine, and missing the plane and losing money).

There have been cases where new-born children of American and Polish parents were denied leaving Poland on US passport, as the child was also a Polish citizen.

Remember that Poland allows dual citizenship, but will not recognize your non-Polish citizenship, and you will be treated as a Polish citizen only. This may result in things like being denied consular protection/assistance from US embassy in Poland.

In order to properly enter and leave Poland and make your stay hassle-free, just order a Polish passport. I am aware that it always more complicated to order it abroad, but you will have no issue with coming to Poland for the next 10 years.