Could it mean my own girlfriend blocked my number?

I sent her a text message using my iPhone and this time doesn't say delivered underneath, so i don't know if she turned off her phone earlier or blocked me

She Has a New Boyfriend that would make her do it.

Try calling her to make sure, if the call goes through and rings then she hasn't blocked you. If he doesn't, she probably just has her phone off. Try contacting her another way (Facebook for example).

My first two phones would sometimes fail delivering messages. It would randomly deliver them hours later and work fine.

Could be a problem with iMessage. Messages sometimes won't send with iMessage active. Mine STILL doesn't work and it started messing up for me at least a year ago.

Yes. Sorry about the loss