Hi Rae,
The imagery of a dog biting your buttocks in your dream, especially given that it was a nightmare with a considerable emotional impact, suggests an experience of being "nipped at" or bothered by a persistent issue in your waking life. Dogs often symbolize loyalty, protection, and intuition, but when they act aggressively in a dream, it can point to a feeling of betrayal, an attack on your sense of security, or inner turmoil regarding someone or something you once trusted. The specific targeting of the buttocks might imply vulnerability or a sense of embarrassment about a situation that's been a pain for you, something that feels personal and perhaps not visible or openly acknowledged by others.
Given the negative feelings associated with this dream, it's possible your subconscious is processing a situation where you feel cornered or pressured, perhaps in a relationship or social context, given your age and life experiences. It could be reflecting the stress of feeling undermined or undervalued. Such a dream encourages personal reflection on these feelings, to understand the source of this metaphorical "bite" in your life. It's important to consider your personal boundaries and whether they are being respected by those around you.