Work place discrimination due to pregnancy?

I currently work full time with a property management company and have so since August 2015. I gave them two months notice about wanting to leave to go home for the holidays and then be a stay at home mom so that they could find my replacement and allow me to train her. I love my job and didn't want to screw them over in any way! I also didn't want to take vacation time and then turn around and take maternity leave and have them hold my position and put strain on the office. I felt my decision was what was best for every....

Fast forward to the holiday season! Every year we have a holiday party for the office where everyone gets a nice gift and gets to get out of the office for a bit. They invite everyone with personally addressed invites....I did not receive one. The party is on the 8th and my last day is the 9th, so because of this the regional management decided I would be the person to stay at the office since they need ONE person there during the party. I was not asked to do this, and it is not normal to ever select someone to be left out with the company. Furthermore, I had three people offer to stay behind in my place so that I could enjoy my last party with my coworkers...regional manager said no, it had to be me. It was also asked if they could bring me back a gift since everyone would receive one. Nope! 

I have done my time with this company and excel at what I do even winning awards and bonuses. Out of the blue I feel like I'm being punished for doing what I believed to be right for the company..... 

I view this party as a reward for a years work, so why am I excluded? 

Is this discrimination? Possibly due to my pregnancy? Do I have any case with HR and if so what is that case? 

I just feel like I'm being wrong and everyone in my office agrees but acts like there's nothing I can do... PLEASE HELP

UPDATE 

I've spoken with the HR department and it absolutely is discrimination since I will be a current employee during the time of the party. I just want to clarify this is a company party, not just a friends party so excluding one person is NOT okay. They are using company funds to thank everyone for the years work and I have been there all year so they have no basis for excluding me.

Also this was never about the gift! It was about the principle of it...Stick with your gut ladies! If something doesn't feel right it probably isn't...