I don't know much about the implant but presume its pretty much the same as the depo as regards hormones, if it is, then its not unusual to takes months for your cycle to get back to normal, even though your first few periods seemed to be on time it doesn't mean your cycle is right again.
If the implant is more like a coil then it is possibly to conceive quite quick! I'd suggest a visit to poundland & grab yourself another test, no good putting it off & off, you will just stress yourself more & possibly delay your period! Maybe wait though till May 26th But don't be disheartened if your not there is always next month X