The first thing you should look at is when and where and what.
1Dates.
2Times
3Witnesses
4What has happened.
If they are saying you are not pulling your weight ask when they came to this idea,inform them that there are correct procedures in place for them to use if they believe you are not doing as you should. Have you at any time been trained and have they got paperwork or training records to prove that you have been trained correctly.
If they have nothing on paper then tell them they can provide training which they are predisposed to provide.If after all that then take a grievance out against the managers sighting the fact that they have not given you the correct training and that the continuing harassment and bullying is unacceptable,use any dates or times that managers have been quiet clearly harassing or bullying you,see if any of your work colleges will stand by you,if a manager comes and says they want to talk to you make sure that someone else is there with you.
Fill out the paperwork with as much information as possible and when you get to the bit where it asks you what do you want to see happen then just say what you are hoping to see to end this type of behavior toward you.
Keep a record of any further problems times and dates make sure you keep a log of what has happened and who was around and like i say make sure you have someone around that will stand as a witness.