Miniature dachshund heat cycle?

I would have her checked out by a vet just to be sure. I had one of my girls desexed and the vet forgot to put on the record that the procedure had been done.
The most likely reason is that she is in heat, but I would hate for her to have an infection and you not have taken her in. Scars are not always visible either, my girl doesn't have a mark on her from when her spay was done, even her ear tattoo had faded so much it's indescernable.

With seasons it doesn't matter how old the dog is they last the same amount of time, 21 days.
Some b*tches can appear to have shorter seasons but they are still capable of becoming pregnant during the entire 21 days so it is important she be kapt away from all males dogs during this time. Don't be lulled into a false sense of security by her being an inside dog, males can smell a b*tch a mile off and often hang around people homes and even attempt to enter the house. The same goes for the b*tch, she may try harder now to escape.

Bleeding generally occurs for around a week but it may be longer or shorter depending on the individual dog. You can purchase products from your local pet store to help deal with the mess, b*tches britches are popular and work similar to a child's diaper (although much less bulky). Alternatively you can just put some old blankets or sheets on your furniture.

At 5 you can still safely have her spayed providing she is in good health, and it would be better for her in the long run as it completely eliminates the chance of uternine infections like pyometra. Pyometra is when the uterus becomes infected and inflamed and actually fills with pus and fluid. It is a fairly common and often undetectable problem it b*tches and can lead to death.

If you have any more question don't hesitate to ask

I agree 100% with Shaz, have her checked to be on the safe side.
The scars can lessen to the point that they are almost undetectable.
Pyometra being high on the list of things that can be avoided by having an older female spayed, there are other things to consider,
One being not having to deal with the mess of heat cylces, also reproductive cancers, and eliminating the hormones that can sometimes cause temperament issues.
And to echoe Shaz again, the things they will do to have their way at that time would surprise you.
The pros far outway the cons when it comes to having pets desexed.
They are generally happier and healthier, and that is a plus for her and your family also.

And lastly, enjoy your new friend and report back, we love updates.