Repeated measurements are negatively correlated

I have a set of data on 9 subjects. A measurement is taken every week for 10 consecutive weeks. Say I would like to compare the measurements taken on week 0 and 14, I believe that I should compare it using paired t-test since these 2 groups are not independent. Is this correct?

I also tried looking at the correlation between the measurements taken at different weeks. The results showed r of about 0.5 - 0.7. On real data, does this make sense? I would expect the r to be closer to 0.8 - 0.9. Also on some comparison, the r showed negative correlation. Isn't this impossible since it is repeated measurements? :confused:

Please help!!