Since you are asking about grammar, I can only presume you are asking about speech, not writing. 

Imagine that you had a parent saying this to their child:

### [ˈjʊwɚːˈpʰɻɪdiˈlɛjʔˈgɪʔn̩ˈhowmˈlæjəsˈnʌjʔˈkʰɪd]

Because the kid got home pretty late last night. Clear enough, right?  That sentence is perfectly grammatical with the contraction as the first word, and its use in the past tense is completely obvious because it sounds different that way — and because it specifies *last night*. So people really do use this—in speech, where nobody bats an eyelash at routine compressions like these.

An uncontracted *you were* would have been quite different: [ˈjuw.wɚː]. It would have had a long vowel and had two words with a geminated [w], not just one run together with its vowel shortened up.

The ***present tense* contraction [jʊɹ]** doesn’t have a long vowel here. But the ***past tense* contraction [ˈjʊwɚː]** would have a longer vowel to start with and then a more drawn-out rhotic at the end to distinguish the tense. It would probably even be perceived as having two syllables with the [w] glide between them, which is why I’ve written it that way.

But **how you choose to *write* that** is  completely up to you: good luck! 

Whatever you choose won’t affect its grammaticality one tiny bit, not even if you were to write that contraction as *Ewwer*. 

But it surnuff might gitcha talk tabout, doncha know. :)