Can a police make up other reasons for initial stop after I'm already arrested?

I followed an unmarked narcotic agent down my street doin 17 mph in a 25 we came to a zig zag in the road and he pulled over to the side..(no blinker) i sat for a few seconds then went around him as soon as i did i seen police lights. He came up to my window and said my window tint was illegal but assistant dea told me it was good, then he asked if i had any weapons i said yea a pistol under the back seat ,he asked me to step out ,i said i dont cosent to any searches he frisked me and found a pipe and weed in my pocket by squeezing it... after he found the weed and i was arrested i accused him of harrasment that he had no reason to pull me over then he said it was cause i didnt use my blinker for a zig zag in the road ,then 5 more min into the search he notices my brake tag and added that too