Has he cheated..

Alice
Me and my boyfriend were laid in bed one sunday evening and he asked me if he could take my car to the shop. Im like 'yea thats fine. Heres £5. Will you get me a bottle of pop please'. Gave him my keys and he left. The shop is about 10minute trip there and back. He left his phone at home and didnt return back untill 2 hours later. When i asked where he had been and he said his ex girlfriends house (they were together on/off 11 years). He got his stuff from the shop but forgot to get my bottle of pop. AND..... gave my £5 to his ex girlfriend!!! He gave me 4 different excuses as to why he was there. I still dont know the truth. He had to reason to go. If he did why did he lie to me and say he was going to the shop. He hasnt been since and i dont think he has spoke to her. He said nothing has happened but why would he lie. Plus that night.. he couldnt get an erection and usually i just need to look at him and hes got one. Its forever playing on my mind and when he is out or late from work i get really nervous. What do you think?