Am I semi-rhotic?

The Original Poster says that they produce /r/ inconsistently after a vowel. Let's look at two words where there is an orthographic R (a written R) after a vowel:

• car - /kɑ:/ in non-rhotic English
• carrot - /kærət/ in non-rhotic English

Notice that in non-rhotic Englishes, there is no /r/ in the first example but there is in the second. This is because in non-rotic English it makes no difference what precedes the R. It could be silence, or a consonant, or a vowel. It doesn't matter. What is important is what comes after the R. If there is a vowel after the R, then it will be pronounced. If there isn't then it won't. Consider the R in car in the following examples:

• car park - /kɑ: pɑ:k/
• car alarm /kɑ:r əlɑ:m/

In the first example, the sound after the R in car is not a vowel, but the consonant /p/. It is therefore not present in the pronunciation.

In contrast, the sound after car in the second example is the vowel at the beginning of the word alarm. Therefore, this R is pronounced.

In conclusion, it is not possible to tell from the Original Poster's observations whether they really are mixing rhotic and non-rhotic pronunciations - we would need more data. However, it is entirely possible that they have learned, or picked up, a non-rhotic variety of English.

It is the sound that follows R that is important in non-rhotic English, not the sound that R follows!