It's unusual, but not unheard of to observe side effects which are cyclic with the hormone therapy injections.
The drug release profile is very different for the two doses. The 12-weekly has a peak release rate at the point of injection which decreases throughout the 12 weeks, but should be high enough in most cases to maintain castrate or lower levels of Testosterone for the whole 12 weeks. The 4-weekly peaks in the middle of the 4 weeks, but the peak is lower than for the 12-weekly. Men can use either the 4-weekly or 12-weekly doses - sometimes the first one is 4-weekly in case you are allergic, but usually 12-weekly injections are used. 4-weekly works out cheaper for the drug, but not if you include the cost of the appointment to inject it.
I'm not sure why, but originally, the 12-weekly was only licensed for men, and women had to always use the 4-weekly (it's also used for estrogen driven cancers), but it does seem the 12-weekly is licensed for women too in some cases now in some countries.