If you replaced your patch earlier rather than later then your estrogen would not have dipped unless the previous patch had come off, so I don't think it can necessarily be attributed to that, however nor do I think you need to panic.
As it has been two years since you last bled, the usual advice would be to make a routine appointment with your GP, however in the setting of hormone therapy provided your cervical screening is up to date and you have no risk factors for endometrial cancer, I personally wouldn't rush to investigate a single episode of spotting.
Vaginal atrophy, which is extremely likely given you are only on 25mcg of estradiol, can also lead to spotting so it may be worth trying some vaginal estrogen if you are happy with your current systemic treatment.
However my approach regarding systemic hormone therapy would be to use a therapeutic dose of estrogen and then manage any bleeding by adjusting the progestogen, rather than reducing the estrogen in response to spotting, so really the question is do you feel well and adequately treated on your current treatment?
