A racing heart at 3 am, sudden flushing, itchy skin, bloating after “healthy” foods, anxiety that feels chemical, and cycles that no longer follow the old rules can all look like perimenopause. They can also look like mast cell activation syndrome, often called MCAS. That overlap can be confusing. Perimenopause changes hormones, sleep, mood, temperature control, digestion, and skin. MCAS affects the immune system and can trigger reactions across many of the same body systems.