Help on cheating girlfriend

OK, so I will explain briefly the story behind my suspicions and then where I'm at in my head a the moment. So My girlfriend of 4 years, have moved into our own house (been there for one year) we have recently had a new next door neighbour move in. One evening we were both supposed to attend a wedding for my Boss. She explained that she didn't want to go so she stayed home. I arrived home early to find that she wasn't at home. She was surprised and told me she was round with the new next door. She came back home drunk at 2am, which is really out of character. The next evening he came round with our other friend, and my girlfriend and him were texting each other in front of us all night. This same event also began to happen every time I was out. She would go round and not tell me she was doing this. I would arrive home unexpected and she would act slightly suspicious. One time she came back and started crying saying she felt out of character. She began to argue a lot with me over various subjects to which I suggested moving back to my moms for a few days to give her space. I arrive back home one morning before work to find the bedsheets in the wash whilst she had left for work. This was very out of character for her. That weekend she went up town with her friend for drinks, when back at home I found a receipt for emergency contraception and the empty box. We hadn't had sex for at least 3 weeks and that was protected. I asked her mother for advice on this, and she said it may have been for her young cousin who was at the same shopping place as she was when they were purchased (i found the receipt)After this I confronted her with my suspicions (not the evidence of contraception) to which she laughed and told me not to be silly, she loves me. After this ordeal she told me to get to know the next doer neighbour more, so I did. He came round for drinks on evening. After a few hours I went upstairs to the computer and when I came down I caught them hugging one another. She told me that I don't trust her as he is going through a rough time with his family, and she was hugging him as a form of endearment. She spent the weekend out of the house (didn't tell me where she was going) and whilst she was out I found a pregnancy test in the bin. This had one thick line and a faint one (which could be the evap line) She has also had two periods since we last had sex. She is constantly texting him and snap chatting him. I just need some advice on what to do next. My current thought is that I don't exactly have enough stick-able evidence to accuse her of cheating, so should I wait it out for more mistakes to appear. Because it seems like she is leaving mistakes and not covering her tracks up very well, so eventually I will probably catch her with more solid evidence if it is going on. As always, there could be other reasons for this behaviour.