Is there any way to see my podcast history?

Go to Settings>Cellular, scroll down and you can see what is using your cellular data. If you haven't reset the results in this list in a while, it may be off, but it should give you somewhere to start.

Yes, the current period is from the last reset. Some people like to reset that at the end of their billing period from their provider to make sure the two somewhat agree. Try resetting it now and watch for a day and see what happens. Also look to see if Wi-Fi Assist is turned on. That will use cellular data if your wi-fi gets a little weak. Turn it off.

There is no way to see a monthly breakdown unless you reset it yourself on a monthly basis. Even if you check data usage with your carrier, they have no idea what the data is used for, so for them it is just data used. This should help you figure it out, and you may find that it really isn't podcasts, it could be something else, once you are able to isolate what you are using it in. Good luck.

A two hour podcast is less than 70MB's.

You do the math how many hours you would have to listen to in 5 days.