Possible loose lid on ferm bucket

I've been brewing for 4 years now so I kind of understand why things happen when I brew, ferment, bottle, etc. I brewed the on Sunday, pitched the yeast at 70 degrees. Ferm bucket did drop to 68 degrees by the next day, no activity. I placed a heating pad near the ferm bucket and raised the temp to 72 (yeast range is 70-75). Took the heating pad away, temp raised on its own to 75 and smells of fermentation.
However, there is no activity in from my blow off tube. I'm wondering if my lid has a small crack in it. If so, is this a huge issue? There should be enough pressure from the gas inside to prevent anything outside getting in right? Or do I replace the lid.
I think this happened to me two years ago but can't quite remember.
Any help is appreciated.
UPDATE: I read numerous threads here about pressure and such during fermentation. I relaxed and had a homebrew. Today I made some beglian candy sugar (didn't have enough on brew day and I knew we could add later) and since I had to open the ferm bucket to add the sugar i swapped out the lid with a new one. Within 5 minutes the lid was bulging due to the pressure inside the bucket. Bubbles through the blow off tube are rolling out now! So if anyone runs into this issue in the future, all will be fine.
I did find a nice 1.5" rip on the back end of the lid. Don't know how we missed that on brew day.

I am surprised that you have been brewing for 4 years and are concerned about a leaky lid. If the beer is covered the only thing that has happened, if the gravity is falling, is that you didn't get to see any bubbling.

BTW 75 degrees it quite high for a fermentation temperature. I would say that everything that happened BEFORE you started raising the temperature was best.

Thanks for the quick reply. I was sort of worried. The vast knowledge on this forum is so great that I figured it was better than ask for others opinions than to guess.
This is a Chimay Blue clone so the yeast range according to Wyest is 70-75. I was thinking of going with a smooth water bath to bring the temp down a degree or two but to stay in that range.
Thanks again.

If I have a lid without a crack should I sanitize and switch them out? I open the lid to take samples so how much risk is there in putting a freshly sanitized lid on?