Disclaimer: Everyone's D/s relationship is different and everyones needs are different. These are just My thoughts
This isn't specifically about aftercare, though aftercare is important and also different for every person/relationship. This is more about subs taking care of themselves, learning to communicate, and knowing when to take a break.
BDSM relationships like D/s can be very intense short bursts in sessions, or they can be more casual long term submission. In either case a sub's care is important, but since I have more experience in long term submission rather than occasional sessions I will be talking more about that.
Much like any form of relationship, shit happens. Whether that is internal or external factors that put stress on your D/s relationship, it is inevitable. Therefore it's important to be able to navigate it and communicate well with your domme.
If you are very busy, have some financial problems, your studies or work are overwhelming at the moment, an emergency has happened, if you need to take a break that is perfectly ok. Tell your domme what is going on (in whatever way you are comfortable), and take a break. Focus on getting through that rough patch, focus on taking care of yourself. You can still interact with and think about your domme, of course, but for a while make sure you are doing what needs to be done to get back into a good place. Most likely, your domme will understand and even encourage the break.
Sometimes our problems are more internal. Our minds like to turn on us, with anxieties or traumas or conditions. BDSM interactions and relationships can be therapeutic, it is true. Sometimes a domme can use their influence over you to help set you to get better. Something to remember though, is that they not your therapist. Using your interactions as an outlet or an escape, using your relationship with your domme to discuss or vent is perfectly fine. But if you really need it, you should consider seeking out an proper therapist to work through your problems. There is no shame in seeking help, for both dommes and subs.
Which brings Me to My next point. I've mentioned before how sometimes doing things for your domme feels easier than doing it for yourself, especially when doing mundane or tedious things. If you find it difficult to see why you should take care of yourself, or be motivated to do so, it can be helpful to do it for your domme/your owner. Should you do it for yourself? Yes, but if doing it for your domme allows you to reframe those needs in your mind and allows you to do them then by all means do it.
If you belong to her, if you are her property, if you are her toy, whatever you consider yourself to be for your domme, then be your best self for her. You can serve her better if you are in a good place both physically and mentally. Can't bring yourself to clean your room and eat because depression is a bitch? Imagine you're doing it for her, taking care of those things for her. Keep forgetting to take your meds? Do it for her, because you know it will help you be your best self for her.
When I say sub self care, I'm talking about not neglecting yourself while you are serving someone else. The power dynamic involved can be thrilling and even addicting, so instead of neglecting yourself use it to reframe what self care means to you. Taking care of what belongs to your domme. Taking care of yourself so you can serve, and so she doesn't need to worry about you not taking care of yourself.
Life is tricky. Self care can be tricky. Why not reframe tricky things to make them less daunting to accomplish?