lack of words to say to my girlfriend

Okay, here goes. Me and my girlfriend are currently in a relationship for 20 months. We are classmates on some of our subjects in the school. Here's the problem. I am a humor type of person to my friends and I talk a lot. When I hang out with my friends, I am confident and comfortable with them. But these past few months, I find it a bit hard to talk to my girlfriend. You know, lack of words to say. Not only me but her also. She is also like me. She is comfortable with her friends but to us, its a bit different. These had never happened on the long previous months where we both jam a lot like going to beaches, hanging out in her boarding house and so on. But it happened from this year's July until now. Whenever she is on a bad mood, I also get in the bad mood and vice versa and thats one of points where I find it hard to talk to her. I try to talk things that she like but her replies are like "ah okay" or simply a nod. Though I can find stuff where I can make her smile and laugh and thats the time we get to talk a lot. Does that mean I have to be funny to her so that we can bond more? I dont know what to do and it usually breaks my happy mood whenever we have a hard time talking to each other. Oh and one more thing, whenever she ask a question about something and I replied back, she told me that she finds it hard to understand my explanation. For me, I think its half true because Im not really into myself because my mind is so occupied in finding ways to talk to her. You know, from a talk-a-lot person into a seemingly-silent-type. I love her so much and Im looking for answers. I missed the old times that we are together and these past few months, it changed.

Guys, please give me some advice. I really need help...