Thin stools, long and thin, sometimes thick and thin at the same time. no blood in stool, sometimes bright red on paper. soft but strain...

Normal Varient: Change in stool caliber as you mention can be a worrisome problem because you think of the possibility of cancer. You do not mention weight loss. Ibs/stress can cause these symptoms. If stool caliber is normal and changes gradually to thin without return to normal, is worrisome. You may have hemorrhoids which explains bright red blood and when they flair could cause these features.

It could just be a : Sign of ibs or colitis.Any alteration in the consistency of the stool or presence of blood be it a stain or gross bleeding deserves an evaluation by a GI specialist which includes colonoscopy. Hope they'll find the cause of your symptoms! take care!