Her2 serum help please!

[Princesswaffle]

Hi, I regularly take the Her2 serum test and have been for two years. I was diagnosed in 2010 at 32with one met to my liver and have been cancer free since my first line of treatment TCH. My question is that my her2 serum tests are above normal now 20ml but my other markers both 15-3 and 27.29, ctc are all normal. The Her 2 serum was normal until 6 weeks ago My fear is that something could be coming back. My her2 serum was 21ml and now it is 20ml three weeks later. Any suggestions to help put my mind at ease. What kind of validity does this test have for metastatic cancer with NED. Thank you very much!
I have the test done every three weeks along with my other tests. I was scanned via MRI to check my liver. I'm do for my 6 month scan in three weeks. My doctor thinks that since I'm on herceptin, tykerb, tomoxofin as well as Lupron and my other numbers are all normal that it is not something to be overally worried about. I on the other hand worry about everything. I feel that the elevation is somewhat high and have no idea what else besides cancer could be causing it to happen. She feel chemotherapy is to toxic at this point when she say I'm Ned and there us nothing showing, but if I really feel strongly we could go down that path.