PEM vs exertion intolerance

I’ve seen a lot of different takes on how soon after exertion PEM starts. I know the typical consensus is after 24-48 hours, but some people say that they experience it immediately while others say that an immediate reaction is exertion intolerance and not PEM. For me, after exerting myself I get extremely fatigued and dizzy, but it lasts for hours or days, slowly going through ups and downs of bettering and worsening until i eventually fully recover. I almost never have a delayed onset of symptoms. Is that PEM or just exertion intolerance? **TLDR;** whats the difference between PEM and exertion intolerance? Can PEM start immediately after exertion?