I'm not a sex addict!

So my girlfriend and I have been dating 3 months and official for 2. We have a good relationship and both feel very in love. I have never felt like this about anyone and I am enjoying how real the feelings are. My concern however is the sex, its not really there. Our average is once every 2-3 weeks...now my girlfriend works a 9-5 as do I. She also has a 4 year old four days aweek . I do not but try to spend as much time as possible when he's there. I feel like I make the time and she makes the excuses. Now maybe I'm being sensitive I dunno. I'm a very attractive girl and have never encountered this problem so I feel super confused. We had an argument about it tonight and she had accused me of "being addicted to sex" ...the kicker for me was when she had mentioned her last relationship (a girl she's still friends with) she had sex with more often in the one month they were together. Why would she even tell me that? We r suppose to be moving in together but I wanted to get some input please hit me with any advice!! Thank u Sincerely - Over worked & under laid