Pee was very diluted when i took the positive tests, but the fmu tests were negatives. is this the hook effect?

Unlikely: It is more likely that the initial test may not been done properly. If you have doubts, it would be prudent to get a blood test for HCG level.

Unlikely: HCG is unlikely to be high enough to produce hook effect in early pregnancy. You may wish to repeat the test and follow the instructions carefully. Alternatively you could get a blood test for HCG level.