In this paper, we propose a novel DynAttGraph2Seq framework to model complex dynamic transitions of an individual user's activities and the textual information of the posts over time in online health forums and learning how these correspond to his/her health stage. To achieve this, we first formulate the transition of user activities as a dynamic attributed graph with multi-attributed nodes that evolves over time, then formalize the health stage inference task as a dynamic attributed graph to sequence learning problem. Our proposed model consists of a novel dynamic graph encoder along with a two-level sequential encoder to capture the semantic features from user posts and an interpretable sequence decoder that learn the mapping between a sequence of time-evolving user activity graphs as well as user posts to a sequence of target health stages. We go on to propose new dynamic graph regularization and dynamic graph hierarchical attention mechanisms to facilitate the necessary multi-level interpretability. A comprehensive experimental analysis of its use for a health stage prediction task demonstrates both the effectiveness and the interpretability of the proposed models.